4782
Chem. Mater. 2006, 18, 4782-4792
Hydrogen in Polar Intermetallics. Binary Pnictides of Divalent Metals with Mn5Si3-type Structures and Their Isotypic Ternary Hydride Solutions E. Alejandro Leon-Escamilla and John D. Corbett* Department of Chemistry and Ames Laboratory-DOE, Iowa State UniVersity, Ames, Iowa 50011 ReceiVed May 24, 2006. ReVised Manuscript ReceiVed July 17, 2006
Studies of the hydrogen-free binary A5Pn3 systems for A ) Ca, Sr, Ba, Sm, Eu, Yb and Pn (pnictogen) ) As, Sb, Bi, reveal four new examples, (Ca,Yb)5Bi3 and Eu5(Sb,Bi)3, among the 16 that form with a hexagonal (P63/mcm) Mn5Si3-type structure. Most of the previous literature on unit cell dimensions for these correspond to those of smaller ternary hydrides (or other materials). Previously reported Ba5As3 and cubic SmAs could not be reproduced. New cubic anti-Th3P4 phases are also found in the Sm-As and Ba-Sb-F systems. Crystal structures have been refined from single-crystal X-ray data for hexagonal Sr5As3H∼1, Ca5Sb3, Ba5Sb3H∼0.7, Sm5Sb3H∼1, Eu5Sb3, Yb5Sb3, and Eu5Bi3. The widths of H solubility regions in the hexagonal parent structures in terms of cell constant changes have been determined for 12 examples, eight of which with more hydrogen further convert into known orthorhombic Ca5Sb3F-type phases. The Sm5Sb3 and Sm5Bi3 members contain trivalent cations according to several criteria. Magnetic susceptibility data are reported for Eu5Sb3 and Eu5Bi3, which contain normal EuII states.
1. Introduction In general, the preparation of materials or modification of existing phases for specific applications requires an understanding of property-structure relationships. Such an understanding is many times limited by uncertainties about the chemical compositions of the compounds. Irreproducibility in the preparation of materials or an inability to obtain products in high yields is often indicative of unsuspected variables or unaccounted reaction components. We have previously reported how adventitious hydrogen impurities can play important roles in the chemistry of alkaline-earth and divalent rare-earth metal pnictides and tetrels with the formula type (AII)5(Pn,Tt)3H for A ) Ca, Sr, Ba, Sm, Eu, Yb and Pn (pnictogen) ) Sb, Bi1,2 or Tt (tetrel) ) Si, Ge, Sn, Pb3,4 in which hydrogen stabilizes numerous ternary phases and structures that otherwise do not exist in the corresponding binary heavy-element systems. In many cases, the principal source of hydrogen has been a hydride impurity in the active metal. For instance, we earlier demonstrated that all eight of the so-called binaries reported between Ca, Sr, Eu, or Yb and Sb or Bi with an orthorhombic β-Yb5Sb3-type structure (Y-type hereafter) were in fact hydrogen-stabilized isotypic ternary compounds with Ca5Sb3F-type (F-type; stuffed β-Yb5Sb3) structures.2 Hydrogen as an unrecognized or ignored impurity has, in sufficient quantity, also often masked the formation of other truly binary phases in various systems or convoluted our (1) Leon-Escamilla, E. A.; Corbett, J. D. J. Alloys Compd. 1994, 206, L15. (2) Leon-Escamilla, E. A.; Corbett, J. D. J. Alloys Compd. 1998, 265, 104. (3) Leon-Escamilla, E. A.; Corbett, J. D. J. Solid State Chem. 2001, 159, 149. (4) Leon-Escamilla, E. A.; Corbett, J. D. Inorg. Chem. 2001, 40, 1226.
understanding of these by yielding mixtures of binary and ternary (hydride) phases. Several A5Pn3 phases have also been reported to crystallize in either the pseudo-binary orthorhombic Y or the hexagonal Mn5Si3 (M-type) type structures or as a mixture of both. The last circumstance has led to the invocation of nonexistent phase transitions to explain the simultaneous formation of two supposedly binary compounds.2 Serious hydride contaminations in fact evidently precluded the discovery of neighboring binary phases such as A16Pn11 (A ) Ca, Sr, Ba; Pn ) As, Sb, Bi),5 Sr31Pb20,6 and A′36Sn23 (A′) Ca, Yb).7,8 Other hydride errors have included Ba5Ga6H2,9 Ba21M2O5H22+x (M ) Si, Ge, Ga, etc., x ∼ 2.0),10 and a family that exists only in a stuffed Cr5B3 structure type, (Ba,Eu,Yb)5Sn3H and (Sr,Ba,Eu)5Pb3H.3,4 The present publication is another aspect of the previous report on binary A5Pn3 compounds, A ) Ca, Sr, Ba, Sm, Eu, Yb and Pn ) Sb, Bi, here concerning the binary phases that exist in a hexagonal M-type (Mn5Si3) structure but which may also exist as isotypic ternary hydride solutions, particularly when the orthorhombic F-type hydride is not stable. An important component of the Mn5Si3 structure is the infinite chains of confacial trigonal antiprisms Mn6/2Si6/2 (A6/2B6/2) that run along 0, 0, z, Figure 1. Many members of this large structural family exhibit distinctive and extensive interstitial chemistries in which third elements, boron or heavier, are bound within the specified antiprismatic inter(5) Leon-Escamilla, E. A.; Hurng, W.-M.; Peterson, E. S.; Corbett, J. D. Inorg. Chem. 1997, 36, 703. (6) Ganguli, A. K.; Guloy, A. M.; Leon-Escamilla, E. A.; Corbett, J. D. Inorg. Chem. 1993, 32, 4349. (7) Leon-Escamilla, E. A.; Corbett, J. D. Inorg. Chem. 1999, 38, 738. (8) Palenzona, A.; Manfrinetti, P.; Fornasini, M. L. J. Alloys Compd. 2002, 312, 165. (9) Henning, R. W.; Leon-Escamilla, E. A.; Zhao, J.-T.; Corbett, J. D. Inorg. Chem. 1997, 36, 1282. (10) Huang, B.; Corbett, J. D. Inorg. Chem. 1998, 37, 1892.
10.1021/cm0612191 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/29/2006
Hydrogen in Polar Intermetallics
Chem. Mater., Vol. 18, No. 20, 2006 4783 The preparations of intermetallic compounds and the dehydrogenation (cleaning) of the starting metals were done according to well-established procedures.2-4 As-received alkaline-earth metals and Yb sealed in tubular Ta containers and in turn in silica jackets were dehydrogenated by heating each while the jacket was maintained under high vacuum (e10-5 Torr), namely, at 650 °C for Ca and at 710 °C for the remainder for ∼12 h or until the vacuum fell below that giving a Tesla coil discharge. All reactions between the elements and metal hydrides were carried out in welded Ta containers heated either under vacuum (to ensure low hydrogen contents) or within evacuated, flamed, and sealed silica jackets. All reagents and products were handled in glove boxes with 0.20.4 ppm (by vol) H2O levels.
Figure 1. ∼[110] view of the confacial antiprismatic A(2)6/2Pn6/2 chain and linear A(1)2Pn6/2 chain in a Mn5Si3-type structure. A(1), dark blue; A(2), blue; Pn, green; and interstitial H, white.
stices (white circles in Figure 1) to give A5B3Zx derivatives, x ) 1 being the limiting stoichiometry11 Such an “octahedral” environment is distinctly more unusual for hydrogen, cationbound tetrahedral sites being preferred when available. The present study has also been extended to include the arsenides. Truly binary M-type compounds exist for 16 of the 18 systems studied. All take up hydrogen, either to transform eventually into the F-type examples or to remain in the M-type phases as A5Pn3Hx. Four new binary examples, namely, (Ca,Yb)5Bi3 and Eu5(Sb,Bi)3, are reported for first time. Moreover, two new examples of anti-Th4P3-type structures exist instead for Sm-As and Ba-Sb-F. Singlecrystal structural data are included for seven examples of M-A5Pn3(H) including the two new binary Eu phases. The experimental protocol used in the syntheses of these phases was as described before.2 X-ray lattice dimensions have again been used as the quickest and best means to differentiate between binaries and ternaries without undertaking direct analyses or neutron diffraction studies. Our results indicate that many literature reports of lattice dimensions assigned to binary M-type members are clearly those of the isopointal hydrides. 2. Experimental Section Syntheses. The materials utilized were Ca, Sr, and Ba (Ae) from APL Engineering (Alfa-Aesar), distilled and sealed in glass; Sm, Eu, and Yb, from Ames Laboratory (>99.99% total, as received); As and Sb, from Aesar, 5-9’s; and Bi, from ORNL, reactor grade. The AH2 phases were prepared by direct reaction of H2 with heated metals in a Mo boat within a vacuum system equipped with a diaphragm manometer.2 All AH2 phases were treated as stoichiometric except for hexagonal SmHg2.5, as verified by their respective cell dimensions. (11) Corbett, J. D.; Garcia, E.; Guloy, A. M.; Hurng, W.-M.; Kwon, Y.U.; Leon-Escamilla, E. A. Chem. Mater. 1998, 10, 2824.
X-ray Diffraction Characterization. Product identification and characterization was done with the aid of EN FR552 Guinier powder pattern measurements and NIST Si as an internal standard (λ (Cu KR1) ) 1.540 56 Å). The phase distributions were estimated from the patterns in terms of equivalent X-ray scattering powers with the aid of patterns calculated for known phases. Although the accuracies of these estimated proportions vary perhaps by (25%, they are internally more precise than that. Importantly, all identifiable phases are reported. Least-squares lattice constants served to differentiate between binaries and ternary hydrides in terms of the customary decrease observed for the latter. These are all listed in Table 1. Note that lattice dimensions from the literature are listed first for each A5Pn3 system. Single-crystal structure data are reported for M-type Sr5As3H∼1, Ca5Sb3, Ba5Sb3H∼0.7, Sm5Sb3H∼1, Eu5Sb3, Yb5Sb3, and Eu5Bi3, all with the Mn5Si3-type structure (P63/mcm, No. 193, Z ) 2). Crystals of these phases were selected with the aid of a low magnification microscope and mounted in thin-walled glass capillaries within a nitrogen-filled glovebox. The quality of these crystals was checked first by means of Laue photographs. Data were collected at room temperature on an EN-CAD4 diffractometer for Ca5Sb3 and on a Rigaku AFC6R instrument for the remainder (ω-2θ scans). Some general data collection parameters are in Table 2, and the calculated distances in the seven compounds are listed in Table 3. (All positional and anisotropic displacement parameters are contained in Supporting Information, Table S1.) Absorption was corrected for all with the aid of three ψ scans collected at χ g 80° and, after isotropic refinement, with DIFABS12 except for Ba5Sb3H∼0.7. Structural refinements (on F) were made with the aid of the TEXSAN package13 (for I/σ(I) g 3 data, 12 variables, and a secondary extinction correction). The results served mainly to confirm the basic nature of and dimensions within the structures. Lattice dimensions secured by least-squares refinement of calibrated Guinier powder pattern data were used in all distance calculations from single-crystal atom parameters. Refinements of heavy atom occupancies gave no indication of any deviations from the A5Pn3 stoichiometries, and full site occupancies were, therefore, fixed in the final least-squares refinements. The F0/Fc listings are available from the corresponding author. Magnetic Susceptibilities. Data were collected from weighed single-phase samples, each held between two fused silica rods that fit snugly inside a 3 mm i.d. tube in a container sealed under He.14 The measurements were made with the aid of a Quantum Design MPMS SQUID magnetometer at 3 T and between 6 and 300 K. The data were corrected for the susceptibilities of both the containers and the atomic cores in the sample. (12) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158. (13) TEXAN, Single Crystal Structure Analysis Software, Version 5.0; Molecular Structure Corporation: The Woodlands, TX, 1989. (14) Guloy, A. M.; Corbett, J. D. Inorg. Chem. 1996, 35, 4670.
4784 Chem. Mater., Vol. 18, No. 20, 2006
Leon-Escamilla and Corbett
Table 1. Synthesis of Hexagonal Mn5Si3-Type Phases and Reactions, Conditions, Product Distributions, and Lattice Dimensions (Å, Å3) of Mn5Si3-Type Products ref (system entry) Ba5Sb3 15 16 18 (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) Arsenides 19 20 (1) (2) 20 21 (1) (2) 21 22 (1) (2) 23 23 (1) (2) 24 25 (1) (2) 26 (1) (2) Antimonides 20 16 (1) (2) 27 16 (1) (2) (3) 28 29 30 31 (1) (2) (1) (2) 32 33 16 (1) (2) (3) Bismuthides (1) (2) (3) (4) 18 16 (1) (2) (3)
loaded compositionsa
Ba5Sb3 Ba5Sb3 Ba5Sb3 Ba5Sb3 Ba5Sb3 Ba5Sb3H0.0 Ba5Sb3H0.5 Ba5Sb3H1.0 Ba5Sb3H2.0 Ba5Sb3F
sc dv (1) sc, (2) dv (1) dv, (2) +H2 dv sc sc sc sc dv
Ca5As3 Ca5As3H2.0
sc dv sc sc
Sr5As3 Sr5As3H2.0 Ba5As3 Ba5As3H2.0 Sm0.968As0.968 Sm0.996As0.800 Sm5As3 Sm5As3H2.0 Eu5As3 Eu5As3H2.0 Yb5As3 Yb5As3H2.0
Ca5Sb3 Ca5Sb3H0.0 Sr5Sb3 Sr5Sb3H0.0 Sr5Sb3H0.5
Sm5Sb3 Sm5Sb3H2.0 Eu5Sb3 Eu5Sb3H2
Yb5Sb3 Yb5.8Sb3 Yb5Sb3H0.0 Ca5Bi3 Ca5Bi3H0.0 Ca5Bi3 + Y Ca5Bi3 Sr5Bi3 Sr5Bi3H0.0 Sr5Bi3H0.5
approximate yieldsd (%) M F other
conditionsb,c
dv sc sc sc dv sc sc sc dv sc sc sc dv sc sc dv sc sc sc dv sc sc sc dv sc sc sc sc sc sc dv sc dv sc sc sc sc dv dv sc dv sc dv dv, q sc sc dv sc sc
1 1 2 3 1 1 1 1 1 1
100 100 97 100 100 93 94 96 94
Ox 5T, Ox 4T, Ox Ox 4T, Ox 80ATP, 15T, 5BaF2
1 1
95 95
Un 5T
1 1
75 50
UN T40, UN ATP 100ATP 100ATP RS RS 100ATP 100RS
1 1 1 1 1 3
98 5
2T 95T
4 3
70 35
30Un 15RS, Un
1 1
98 95
1 1 1
95 100 40
3 3 1 1
80 95 98
2T 5 3T, Ox 60
98
1 1 1
90 95 90
2
1 1 1 2
5 6 30 90
95 92 70 8
1 1 1
95 95 43
3 55
20ATP 5ATP Ox Ox
5T, Un 5T 8T
a
lattice dimensions b Vcell
9.969(3) 9.964(3) 9.97(1) 9.977(1) 10.018(2) 10.023(2) 9.968(3) 10.020(1) 9.974(2) 9.972(1) 9.973(1) 9.972(2) 10.5686(4)e
7.733(6) 7.694(4) 7.73(2) 7.727(2) 7.788(4) 7.786(3) 7.680(6) 7.788(2) 7.745(4) 7.673(2) 7.671(2) 7.672(3)
8.43 8.479 8.4885(3) 8.4412(8) 8.942 8.93 8.930(3) 8.917(4) 9.49 9.973(1)e 9.9646(5)e 9.9493(8)e 5.9205(4) 5.9097(2) 8.8361(2)e 5.9076(3) 8.8526(3) 8.8646(9) 8.866(2)
6.75 6.844 6.8436(5) 6.774(3) 7.355 7.32 7.333(7) 7.302(6) 7.90
665.5(7) 661.5(5) 665(2) 666.1(3) 676.9(5) 677.4(4) 660.9(6) 677.2(2) 667.3(5) 660.8(3) 660.7(2) 660.8(4) 1180.4(1)
c/a 0.776 0.772 0.775 0.774 0.777 0.777 0.770 0.777 0.776 0.769 0.769 0.770 1.000
7.0376(4) 7.0811(4) 7.057(3)
415 426.1 427.05(4) 418.0(2) 509.3 506 506.5(6) 502.8(6) 616.1 992.0(2) 989.4(1) 984.9(3) 206.75(8) 206.39(4) 689.90(4) 206.18(3) 477.64(7) 481.9(2) 480.5(3)
0.801 0.807 0.806 0.802 0.822 0.820 0.821 0.819 0.832 1.000 1.000 1.000 1.000 1.000 1.000 0.795 0.799 0.796
8.480 8.4879(4) 8.4438(8)
6.671 6.7116(6) 6.5825(8)
415.4 418.76(6) 406.44(9)
0.787 0.791 0.780
9.024 9.0321(3) 9.0312(3) 9.0315(2) 9.496(5) 9.5037(5) 9.510(1) 9.5035(5) 9.510(3) 9.100 8.99 9.18 8.992(1) 9.113(2) 9.1043(4) 9.4183(4)
7.057 7.0280(8) 7.0254(4) 7.0251(3) 7.422(5) 7.4095(8) 7.426(2) 7.4108(7) 7.422(4) 6.40 6.38 6.40 6.320(1) 6.381(2) 6.3451(5) 7.2124(4)
497.7 496.5(7) 496.24(4) 496.25(3) 579.6(7) 579.6(1) 581.6(2) 579.6(1) 581.4(5) 459 446.5 467.1 442.5 458.9(2) 455.47(6) 554.06(8)
0.782 0.778 0.778 0.778 0.782 0.780 0.781 0.780 0.780 0.703 0.710 0.697 0.703 0.700 0.697 0.766
8.995 8.997(1) 9.0344(2) 9.0357(4) 9.0366(6) 9.0292(4)
6.870 6.872(1) 6.9112(4) 6.9047(7) 6.9044(9) 6.8985(7)
481.4 481.7 488.52(4) 488.21(7) 488.27(9) 487.06(7)
0.764 0.764 0.765 0.764 0.764 0.764
9.172(1)
7.145(2)
520.6(2)
0.779
9.1757(3) 9.63(1) 9.651(2) 9.6554(6) 9.647(1) 9.654(1)
7.1444(4) 7.63(2) 7.523(5) 7.5321(8) 7.523(2) 7.527(2)
520.93(4) 614(2) 606.8(5) 608.1(1) 606.4(2) 607.5(2)
0.779 0.792 0.779 0.781 0.780 0.780
2T 2T 2T, Ox Ox Ox
Hydrogen in Polar Intermetallics
Chem. Mater., Vol. 18, No. 20, 2006 4785
Table 1. (Continued) ref (system entry)
loaded compositionsa
Bismuthides (continued) 20 18 16 (1) Ba5Bi3 (2) Ba5Bi3H2.0 34 (1) Sm5Bi3 (1) Eu5Bi3 (1) Yb5Bi3 (2) Yb5Bi3
conditionsb,c sc sc sc dv sc sc dv dv dv dv, q
M
approximate yieldsd (%) F other
1 1
95 85
Ox 10T, Ox
3 1 3 2
30 97 25 85
65ATP Ox 75 15
lattice dimensions b Vcell
a 10.10 10.13(1) 10.098(2) 10.1556(2) 10.096(1) 9.30 9.268(3) 9.5551(3) 9.1866(2) 9.1852(2)
7.78 7.79(2) 7.768(3) 7.8912(2) 7.766(2) 6.48 6.456(3) 7.3256(5) 7.0401(5) 7.0391(4)
c/a
687 692(2) 686.0(4) 704.84(2) 685.4(3) 485 480.2(3) 579.22(5) 514.54(6) 514.31(4)
0.770 0.769 0.769 0.777 0.769 0.697 0.697 0.767 0.766 0.766
a Reactions designated Ae Pn H b 5 3 2.00 utilized previously dehydrogenated metal. sc ) Ta container in sealed SiO2 jacket; dv ) dynamic high vacuum about Ta container; q ) quenched. c Conditions: 1, 1100 °C for 2-4 h, 4-10 °C/h to 650 °C; 2, 1100 °C for 2 h then quenched to RT; 3, same as 1 but 1150 °C initially. d Estimated from Guinier powder patterns. Structure types: M, hexagonal Mn5Si3; F, orthorhombic Ca5Sb3F; T, tetragonal Ca16Sb11; Ox, A4Pn2O (anti-K2NiF4); ATP, anti-Th3P4; RS, NaCl; Un, unidentified. e Data for cubic ATP phase.
Table 2. Crystallographic Data for Hexagonal A5Pn3Hx (P63/mcm, Z ) 2) compound
Ca5Sb3a
Sr5As3H∼1
Ba5Sb3H∼0.7
Sm5Sb3H∼1
Eu5Sb3
Eu5Bi3
Yb5Sb3
lattice parameterb a (Å) lattice parameterb c (Å) volume (Å3) density calcd (g/cm3) octants collected transm. coeff. range absorption coeff. (Mo KR) (cm-1) no. reflect. meas. indep. (obs., I > 3σ(I)) Rave (%) R/Rw (%) goodness of fit max/min in ∆F map (e- Å-3)
9.0312(3) 7.0254(4) 496.24(4) 3.785 h,k,(l 0.875-1.262 106.2 1057 208 (186) 3.43 2.5/3.5 1.717 1.27/-1.25
8.917(4) 7.302(6) 502.8(6) 4.385 h,(k,l 0.680-1.272 352.7 1013 213 (148) 9.08c 2.5/2.6 1.339 0.73/-1.21
9.977(1) 7.727(2) 666.1(3) 5.244 (h,k,l 0.554-1.000 205.1 1348 277 (214) 4.18 1.4/1.7 1.766 0.85/-0.61
9.1043(4) 6.3451(5) 455.47(6) 8.153 h,(k,l 0.929-1.122 405.7 1733 218 (173) 2.88 1.7/2.0 1.511 1.99/-1.12
9.4183(4) 7.2124(8) 554.06(8) 6.743 (h,k,(l 0.909-1.097 350.9 2073 182 (177) 3.64c 1.8/2.4 1.393 1.34/-1.61
9.5551(3) 7.3256(5) 579.22(5) 7.951 (h,k,(l 0.719-1.218 719.0 2199 247 (187) 5.97 1.6/1.9 1.158 1.51/-1.39
9.0366(6) 6.9044(9) 488.27(9) 9.323 h,(k,l 0.760-1.198 630.5 1006 212 (130) 9.45 c 4.1/4.0 1.251 3.18/-3.90
a Data from Enraf-Nonius CAD4; remainder are from Rigaku AFC6R diffractometer. b Lattice parameters from Guinier powder patterns. c For I/σ(I) > 3 data.
Table 3. Interatomic Distances (Å) in A5Pn3Hx Phases with Mn5Si3-Type Structure atom(1)-atom(2) Pn-A(1) Pn-A(2) a Pn-A(2)b Pn-A(2) Pn-Pn A(1)-A(1) A(1)-A(2) A(2)-A(2)a A(2)-A(2) a
6× 2× 1× 2× 2× 2× 6× 2× 4×
Ca5Sb3
Sr5As3H∼1
Ba5Sb3H∼0.7
Sm5Sb3H∼1
Eu5Sb3
Eu5Bi3
Yb5Sb3
3.2992(2) 3.0879(6) 3.237(2) 3.7232(9) 4.0436(6) 3.5127(2) 3.857(1) 3.949(4) 4.188(1)
3.301(1) 3.061(2) 3.198(2) 3.863(3) 4.132(3) 3.651(3) 3.865(2) 3.859(3) 4.277(3)
3.6324(4) 3.4279(6) 3.571(1) 4.113(1) 4.427(1) 3.864(1) 4.2693(6) 4.328(1) 4.6010(9)
3.2372(3) 3.0948(6) 3.3072(9) 3.4108(4) 3.7798(8) 3.1725(3) 3.8238(4) 3.936(1) 3.9024(4)
3.4241(3) 3.2260(7) 3.359(1) 3.8263(6) 4.163(1) 3.6062(4) 4.0024(5) 4.140(1) 4.3265(6)
3.4725(2) 3.2756(5) 3.419(1) 3.8952(4) 4.2191(6) 3.6628(3) 4.0732(6) 4.166(2) 4.3819(6)
3.2821(9) 3.090(2) 3.238(4) 3.667(1) 3.987(3) 3.4512(3) 3.846(1) 3.943(4) 4.134(1)
Distance within confacial units, normal to the c axis. b Interchain distance.
3. Results and Discussion Hydrogen plays an important and defining role in the chemistry of A5(Pn)3(H) (Pn ) As, Sb, Bi) phases for the alkaline-earth and divalent rare-earth metals as well as in the corresponding tetrels with Si-Pb.3,4 The binary A5Pn3 pnictides of nominally divalent A metals with Mn5Si3 (Mtype) structures have by simple valence electron counting a formal excess of two valence electrons per cell (Z ) 2), and thence a presumed metallic character if normal closed-shell configurations (oxidation states) are assigned to classic isolated Pn3- anions. Accordingly, our studies of these phases had their genesis in our inability to rationalize the anomalous semiconducting behavior of the supposed “binary” M-type Ba5Sb3 in a system in which the corresponding orthorhombic F-type hydride does not form. Temperature-dependent fourprobe direct current resistivity measurements on pressed and annealed pellets of single-phase M-type Ba5Sb3 by Wolfe15 yielded a band gap of ∼0.30 eV and a room-temperature
resistivity of ∼0.49 Ω cm. We first sought unique structural features in a larger or distorted cell that might account for the disappearance of the nominally free electrons. However, overexposed precession and Weissenberg photographs of single crystals did not provide any evidence of other structures, and structural refinement of single crystals of hexagonal Ba5Sb3 from test reactions (discussed below) provided no additional information or help regarding atom occupancies or unusual features. On the other hand, Hurng and Corbett16 provided a very important piece of information, that fluorine quantitatively converted M-type Ca5Sb3 and Ca5Bi3 into single-phase Ca5Sb3F and Ca5Bi3F with the orthorhombic stuffed β-Yb5Sb3- (or, better, Ca5Sb3F-) type structures. Such a discovery together with clear variabilities in reported lattice dimensions for such antimonides pressed us to reconsider the possibility of impurities in “Ba5Sb3”. (15) Wolfe, L. G. M.S. Thesis, Iowa State University, Ames, IA, 1990. (16) Hurng, W.-M.; Corbett, J. D. Chem. Mater. 1989, 1, 311.
4786 Chem. Mater., Vol. 18, No. 20, 2006
The presence of inadvertent impurities that were either in very small amounts or transparent to X-ray diffraction was seriously considered, and the list of possibilities was quickly narrowed down to hydrogen. (The alkaline-earth metals had a bad but little known reputation for their ready retention of hydrogen from, say, their reactions with water vapor.17) Two synthetic procedures were devised in order to validate our suspicions about the unrecognized presence of hydrogen in the Ba-Sb system. (I) To minimize hydrogen content, Ta tubing with the elements welded within was placed inside fused silica tubes connected to a high-vacuum line, and the entire reaction cycle was performed under dynamic high vacuum (dv condition). This is effective because Ta becomes relatively transparent to H2 above ∼550 °C. (II) To control hydrogen contents better, active metal reagents that had previously been so dehydrogenated plus known binary hydrides and pnictogens held within welded Ta containers were in turn sealed inside well-flamed and evacuated fused silica ampoules (sc conditions). To avoid cross-contamination between reactions, each A5Pn3Hx composition was reacted inside a separate SiO2 jacket. Measured amounts of hydrogen were added, but the hydrogen contents of the products were not quantified directly; instead, the binding of hydrogen was deduced from variations in their lattice dimensions, first for systems in which the ternary F-type structure was not an alternative and later for systems in which both structure types could exist in equilibrium. Previous manometric and structural studies have established that hydrogen is released and taken up by these phases at the higher and lower temperatures used in these studies, respectively (Table 1). Ba5Sb3 Results. Those that define the Ba5Sb3 system will be described first in some detail, and afterward, the effects of hydrogen in other M-type systems will be described as well. All of the representative results are outlined in Table 1. In each case, lattice dimensions of M-type phases from the literature are given first. As tests of the impurity hypothesis, reactions of Ba5Sb3 compositions, entries 1 and 2 in Table 1, loaded with as-received reagents were heated in parallel at 1100 °C, one under sc (sealed container) and the other under dynamic vacuum (dv) conditions. In this instance no F-type structure in known, and both gave singlephase, hexagonal M type products; however, shifts in the low angle diffraction lines were evident to the naked eye. Cell dimensions of the dv product were significantly larger, corresponding to a cell volume difference of 1.6%, even though in principle the hydrogen in the sc result came only from water desorption from the silica shell.35 Such general (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27)
Peterson, D. T. J. Met. 1987, 39 (5), 20. Eisenmann, B.; Deller, K. Z. Naturforsch. 1975, 30b, 66. Hu¨tz, A.; Nagorsen, G. Z. Metallkd. 1975, 66, 314. Bruzzone, G.; Franceschi, E.; Merlo, F. J. Less-Common Met. 1978, 60, 59. Better, B.; Hu¨tz, A.; Nagorsen, G. Z. Metallkd. 1976, 67, 118. Li, B.; Mudring, A.-V.; Corbett, J. D. Inorg. Chem. 2003, 42, 6940. Taylor, J. B.; Calvert, L. D.; Despault, J. G.; Gabe, E. J.; Murray, J. J. J. Less-Common Met. 1974, 37, 217. Taylor, J. B.; Calvert, L. D.; Utsunomija, T.; Wang, Y.; Despault, J. G. J. Less-Common Met. 1978, 57, 39. Wang, Y.; Calvert, L. D.; Gabe, E. J.; Taylor, J. B. Acta Crystallogr. 1978, 34B, 2281. Ono, S.; Despault, J. G.; Calvert, L. D.; Taylor, J. B. J. Less-Common Met. 1970, 22, 51. Martinez-Ripoll, M; Brauer, G. Acta Crystallogr. 1973, 29B, 2717.
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cell expansions on hydride removal reflect enhanced repulsions between the cations that define the interstitial cavities. In other systems in which the orthorhombic F-type is an alternative structure for the tertiary hydride, the change between powder patterns of products of sc and dv reactions was dramatic, as reported for the (Ca,Yb)-Sb and Sr-(SbBi) systems.2 In all of these systems, the truly binary phases exhibit M-type structures. The cell dimensions of the sc product “Ba5Sb3(H)” are close to those reported by Wolfe.15 The dimensions of this compound reported by Eisenmann and Deller18 are included in Table 1 for historical reasons, although their precision is lower. Involvement of hydrogen in the single-phase Ba5Sb3 system was concluded once cell dimensions of the binary M materials had been repeatedly reproduced, entries 3 and 5. A reversible hydrogenation of single-phase M-Ba5Sb3 was also established, as when powder from an sc reaction, entry 1, was subsequently heated under dv conditions at 1100 °C, entry 3, or with the same conditions applied in reverse order, entry 4. Thus, the extremes of hydrogen absorption in M-type Ba5Sb3 are reflected in a difference in cell volumes of ∼16.4 Å3, or 2.4%. The powdered binary product from a dv reaction 2 in a clean Mo boat was also annealed at 300 °C for 3 h under an initial 685 Torr of H2 gas, where after calculations from the amount of hydrogen consumed indicated that the nearly stoichiometric phase had been formed, Ba5Sb3H∼0.92. In this case, the lattice dimensions and cell volume were even smaller than those from previous experiments at higher temperatures. Experiments with presumably controlled hydrogen contents utilized cleaned Ba metal and BaH2 to give the compositions Ba5Sb3Hx [(5 - x)Ba + (x/2)BaH2 + 3Sb] for x ) 0, 0.50, 1.0, and 2.0, entries 6-9, respectively. A plot of the hydrogen composition loaded (x) versus the product cell volumes is shown in Figure 2, the cell volume of the binary phase obtained under dv conditions being included for reference. Important observations from this are that (a) the sc reaction without loaded BaH2, entry 6, gave a cell volume about 1.5% smaller than for the dv products (entry 5); (b) according to lattice dimensions, hydrogen saturation (probably for stoichiometric x ∼ 1.0) was already achieved for the nominal Ba5Sb3H0.5 composition, with no further change for reactions with more hydrogen, entries 8 and 9 (x ) 1 and 2); and (c) in this system, hydrogen from the dehydration of the silica jacket35 at 1100 °C for 2-4 h is already significant for a sample loaded without hydrogen (x ) 0). Water evolved in this way presumably decomposes on the Ta surface to produce hydrogen atoms that readily diffuse through the container, but oxygen also diffuses (28) Borzone, G; Borsese, A.; Delfino, S.; Ferro, R. Z. Metallkd. 1985, 76, 208. (29) Abdulsaljamova, M. N.; Abulkhaev, V. D.; Levitin, R. Z.; Markosijan, A. S.; Popov, V. F.; Yumaguzhin, R. J. Less-Common Met. 1986, 120, 281. (30) Sadigov, F. M.; Shahguliev, N. S.; Aliyev, J. Less-Common Met. 1988, 144, L5. (31) Abulkhaev, V. D. Inorg. Mater. 1992, 28, 64. (32) Bodnar, R. E.; Steinfink, H. Inorg. Chem. 1967, 6, 327. (33) Abulkhaev, V. D. Inorg. Mater. 1999, 35, 431. (34) Sadigov, F. M.; Geidarova, F. A.; Aliev, I. I. Russ. J. Inorg. Chem. (Engl. Transl.) 1988, 33, 1238. (35) Rustad, D. S.; Gregory, N. W. Inorg. Chem. 1982, 21, 2929.
Hydrogen in Polar Intermetallics
Figure 2. Cell volumes of products of Ba5Sb3Hx syntheses run in welded Ta containers within SiO2 jackets (sc) as a function of x (loaded H per f.u.). The dv point marks the result obtained under dv, showing that hydrogen is a contaminant in the sc reactions.
through, probably at a lower rate, to form the stable phase Ba4Sb2O, which then upsets the loaded stoichiometry and causes the formation of the Ba-poorer Ba16Sb11 (Ca16Sb11 or T-type) phase as well, Table 1. The lattice dimensions achieved from reactions 5-8 also demonstrate that all previous reports of binary “Ba5Sb3”15,16,18 pertained to the ternary hydride. The heavy atom structure of the ternary hydride Ba5Sb3H∼0.7 was refined from X-ray diffraction data collected from a needle-like crystal from the entry 1 reaction, Tables 2 and 3. The formula deduced for the data crystal is ∼Ba5Sb3H0.7 according to its cell volume, with the assumption that the two composition extremes are those represented by the volumes from the dv and hydrogen-saturated Ba5Sb3H1.0 (x ) 1 or 2) reactions and that Vegard’s law applies. This is still an approximation; the stoichiometric x ) 1 limit for H is reasonable, but it has not been confirmed. The diffraction data were refined without complication starting with the atom positions of the model structure. For obvious reasons, the hydrogen atom was not included in the refinement, but according to many other M-A5B3Z phase studies11 it is expected to lie at 0,0,0. Important features of the structure revolve about the Ba2 atoms that define the trigonal antiprismatic interstitial cavity, Figure 1. Interatomic Ba2-Ba2 distances in the triangular face normal to z, d ) 4.33 Å, are 0.27 Å less than those with a principal component parallel to z, d ) 4.60 Å. Such an elongation is probably at least in part determined by the short Ba1-Ba1 contacts in the commensurate linear chain of Ba1 along 2/3,1/3, z, and so forth, for which d ) 3.86 Å (c/2), about 1.5% less than twice the metallic radius of the element, d ) 3.96 Å,36 although there is no particular reason why this particular bond order should pertain. The assumed Ba2-H distance to the center of the antiprism, ∼3.158 Å, is appreciably greater than that predicted from the sum of crystal radii37,38 for Ba2+ and H-, about 2.71 Å. (An off-center H- is perhaps likely in a large cavity, at least in a dynamic mode.) A reaction loaded as Ba5Sb3F produced a product with an anti-Th3P4-type structure (entry 10) rather than an F-type (36) Pauling, L. Nature of the Chemical Bond; Cornell University Press: Ithaca, 1966; pp 93, 403. (37) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751.
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product. It is conceivable that the similarly sized but harder F- is better bonded in smaller tetrahedral cavities with the larger Ba, although there is more to the stability comparison than just this point. A Zintl formula Ba4Sb2.5F0.5 is contemplated for this new phase, consistent with the other products. The binary Ba4Sb3 is evidently unstable. Other A5Pn3 Systems. It was considered essential to establish whether other M-(AII)5Pn3 systems behave similarly in the presence of hydrogen. Therefore, the study was extended to all other alkaline-earth (Ca-Ba) and nominally divalent rare-earth metal pnictides, arsenides included, namely, for AII ) Ca, Sr, Ba, Sm, Eu, Yb and Pn ) As, Sb, Bi, especially regarding the M-type precursors in systems in which Y-type products had been reported. For most of these systems, the survey reactions were limited to two per system; one in the absence of hydrogen (dv) and one in its presence (sc), loading the compositions A5Pn3H2.0 to ensure that the products were somewhat closer to saturation. Arsenides. With the exception of Ba and Sm, the binary and ternary arsenic systems exhibit only M-type structures. Reactions employing as-received elements under dv conditions gave the hexagonal phases in high yields. Objective comparisons of our cell dimensions with those from previous reports were not always straightforward inasmuch as many of the latter lacked standard deviations and, in many cases, any details of the syntheses. Reactions of elemental As involved some difficulties because it corroded the Ta container at sufficiently high temperatures. Many of the Ta containers became brittle after the reactions of A and As, with or without a binary hydride. Although not practiced, pre-reaction of these at lower temperatures in silica would lower the As activity and probably eliminate this effect; the intermediate tantalum arsenides should be reduced by metal A if diffusion or transport of the latter is present. Ca-As. M-type materials were obtained in high yields from both dv and sc reactions, entries 1 and 2. The usual cell volume decrease in this case is about 2.12%. Earlier reports of this phase19,20 seem to correspond to about the two extremes of the homogeneity region observed with hydrogen. Sr-As. The dv reaction, entry 1, gave an M-type phase as the main product, but the sc reaction with an excess of hydrogen, entry 2, produced a 95:5 mixture of M and the tetragonal T (Ca16Sb11) type phases. The latter was probably the result of the stoichiometry imbalance originating with the presence of excess SrH2 with the loaded stoichiometry Sr5As3H2.0. In general, all reactions carried out at this stoichiometry gave such a mixture of products. The unknown phase appearing in both products was not pursued. The interstitial hydride had a smaller cell, by 0.73%. One set of lattice dimensions reported eartlier20 (without standard deviations) for the “binary” appears to be larger than the range of values found here. Some pioneering work on such intermetallics is known to pertain to M-type compounds stabilized by heavy atom impurities.11 The refined structure of a crystal from reaction 2, Sr5As3H∼1, revealed only standard features. The Sr2-Sr2 distances in the trigonal prismatic array are (38) Corbett, J. D.; Marek, G. S. Inorg. Chem. 1983, 22, 3194.
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about 10.8% larger along the linear chain direction. This phase shows the largest elongation of the antiprism among the structures reported here, probably a reflection of the relatively small anion. Ba-As. In 1968, Better and co-workers21 reported a singlecrystal X-ray-based structure for M-Ba5As3, but they provided almost no information about its preparation at 1400 °C or the refinement. Our attempts to reproduce this product under both dv and sc conditions at 1100 °C failed (entries 1 and 2); rather, only products with the cubic anti-Th3P4 type structure were obtained. Stabilization of the former by heavier impurity atoms may have been responsible. Our hydrogen reaction produced a 0.45% shrinkage of the cubic cell, suggestive of a hydrogen interstitial. A Zintl phase composition of Ba4As2.5H0.5 is arguable; note that its Ba:As ratio is very close to what was loaded, consistent with the high yield. Refinement of the structure from the dv product gave a composition of Ba4As2.8(1);39 the Zintl phase composition for a structure (without any close As-As contacts) would be Ba4As2.67. In a later study, an apparently semiconducting single-phase product was obtained from a loaded composition Ba4As2.67 that had been reacted in a sealed Nb-silica container system, and refinement of single-crystal data yielded the composition Ba4As2.60(2),22 which is statistically indistinguishable from the less precise earlier result. Still, the lattice constants of the two binary products differ by 7σ, and the nominally smaller As content in the last compound could lead to greater cation-cation repulsions and the larger cell. The same is true for tetragonal Ca11Sb10-x (P42/mnm)39 which is formed when the synthesis of the well-known Ca11Sb10 (Ho11Ge10-type, I4/mmm)40 is loaded slightly low in Sb. The absence of 8.6% of all Sb atoms in the very similar structure of the former also leads to a cell volume increase of about 0.54%. Sm-As. Material with an M-type structure did not form in either reaction; rather, the binary dv conditions, entry 1, again yielded an anti-Th4P3 structure. There are no previous reports of this phase. The hydrogen reaction gave a rock salt (NaCl) type structure. The reported Sm-As phase diagram contains only one phase with rock salt structure,41 and so this probably pertained to a hydride; stabilization by other small impurity atoms seems unlikely in view of the similarities of the lattice constants. The reported cell volumes24 decrease in parallel with the amount of As (calculated from density measurements). Our hydride was about 0.66% smaller than reported for the stoichiometric phase, which seems to contradict the comments above about the Ba-As phase. However, it is possible that arsenic vacancies in our compound were partially filled by hydride, giving a cell contraction. Eu-As. There are two reports of hexagonal phases near the Eu5As3 composition from the same source, the M-type as the low temperature form and a Ca5Pb3-type (Ca17Sb9, a x3 × x3 supercell deformation of M) as a high temperature (39) Leon-Escamilla, E. A. Ph.D. Dissertation, Iowa State University, Ames, IA, 1996. (40) Deller, K.; Eisenmann, B. Z. Naturforsch. 1976, 31b, 29. (41) Ono, S.; Hui, F. L.; Despault, J. G.; Calvert, L. D.; Taylor, J. B. Solid State Commun. 1971, 25, 87.
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form.25,26 The latter required a slight excess of Eu. The M type was obtained in the absence of hydrogen, entry 1, with no reflections that might be associated with a superlattice, although a trace of a T-type phase was present. Our cell dimensions are very similar to those in earlier reports. On the other hand, a tetragonal T phase was the main product when an Eu5As3H2 composition was used, entry 2. The latter may result from a stoichiometry upset from the loading (unseen EuH2), but no additional reactions were performed to clarify this situation. Questions about a possible hydrogen stabilization of Eu16As115 were eliminated after this compound was replicated in high yield under dv conditions. Yb-As. Problems of incomplete reactions between these elements at 1150 °C were solved by increasing the temperature to 1300 °C. As in the Sr-As system, the products included a phase that could not be identified. Judging from the range of lattice dimensions, the two M phases exhibit the largest range of hydrogen uptake found, with a cell volume decrement of 2.94% for the hydride. Although magnetic susceptibility measurements were not made, the c/a ratios for both cells (0.78-0.79) suggests that Yb is divalent in both, axial ratios for the trivalent metal antimonides and bismuthides lying close to 0.700.29,42 Lattice dimensions for the previously reported M-phase26 are probably those of a partial hydride, although the two values do not scale proportionately. Antimonides. Among the 5-3 antimonides, the hydrides for both Ba and Sm crystallize with M-type structures. The remaining systems yield F-type hydride structures.2 Ca-Sb. Single M-type phases were obtained from dv reactions, entry 1. Use of cleaned Ca metal under sc conditions led to a mixture of M- and F-type structures, entry 2, with the hydrogen necessary to form the latter presumably originating with dehydration of the silica tube.35 Yields of the F phase increased significantly when CaH2 was added. Note that the dimensions of the M and F phases when both are present are those of the hydrogen-saturated and hydrogenpoorest phases in equilibrium. These indicate that the M structure has a relatively very narrow homogeneity range for hydrogen uptake, invariant within the precision of these measures, before it converts to the F-type. Some results of the structural refinement of binary M-Ca5Sb3 are listed in Tables 2 and 3. The standard features of the structure are present, for example, short cation-cation contacts on the linear chain, d(Ca1-Ca1) ) 3.5127(2) Å, and an elongation of d(Ca2-Ca2) along the antiprismatic cavity by ∼6.0%. The synthesis for the first-listed literature data20 was carried out in a sealed iron crucible, and impurities from there may have been responsible for the divergent lattice dimensions and the large c/a value. However, hydride (or fluoride) was present too inasmuch as they also obtained the ternary F (Y) phase. Sr-Sb. As with Ca, strontium forms an M-type phase in the absence of hydrogen, and reactions with controlled amounts of SrH2 gave mixtures of the M and F type cells. The hydride stability range of this M-type material is smaller than for the Ca-Sb counterpart, and so cell dimensions of (42) Yoshihara, K.; Taylor, J. B.; Calvert, L. D.; Despault, J. G. J. LessCommon Met. 1975, 41, 329.
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the binary are in moderately good agreement with previous reports. Sm-Sb. The samarium antimonide (as well as the bismuthide, below) shows a distinctively different chemistry from that of the earlier alkaline-earth metals or of the more dominantly divalent Eu and Yb analogues to follow. The M-type structure is stable without or with hydrogen, entries 1 and 2, respectively. The small variation from the binary to the hydride, with ∼0.74% volume decrement, indicates that a modest amount of hydrogen can be incorporated. Most reports in the literature are not very meaningful. With c/a ratios of about 0.70, the Sm ions in both phases might be judged to be trivalent inasmuch as M structures of consistently trivalent rare-earth-metal antimonides and bismuthides have been reported to have such proportions,29,42 whereas nominal divalent cations in this structure type generally give ratios greater than 0.76 (Table 1). The cell volumes for these two samples also give qualitative support for their trivalent condition, values of 455-459 Å3, being very distinctly less than for the strontium analogues, around 581 Å3, even though the standard crystal radii for the two divalent cations differ by only 0.01 Å.37 Similarly, the disparity in average cationSb distances between Sm5Sb3H∼1 and Eu5Sb3, 0.21 Å (Table 3), is striking inasmuch as there is only a ∼0.02 Å difference in their standard divalent crystal radii, but the metallic radii for the two elements differ by a reasonable 0.23 Å. The particularly short d(Sm1-Sm1) in the linear chain in the hydride, 3.17 Å (Table 3) versus 3.25 Å for the estimated metallic diameter,36 is also noteworthy. In the M structure type, confacial (distorted) antiprisms of Sb about Sm1 are commensurate with trigonal antiprisms of Sm2 around Z (the cavity), Figure 1, the former leading to considerably shorter Sm-Sm distances of c/2. The magnetic susceptibility behavior of these two samples below ∼170 K is rather complex. Although the studies are incomplete, magnetization measurements as a functions of field suggest that their increase with decreasing T between 175 and about 140 K reflects antiferromagnetic ordering whereas a second large maximum around 100 K reflects some sort of ferromagnetic ordering. These effects are larger and at lower temperatures in the hydride, with the second having an appreciably larger maximum at about 55 K. Abdusalyamova and co-workers43 earlier commented on a probable mixed valency in the binary compound according to unpublished magnetic data, but the presence of possible impurities such as hydrogen was not considered. Eu-Sb. This system illustrates well how an unrecognized impurity that is invisible to X-rays can obscure the stability of a truly binary phase. M-Eu5Sb3 was obtained for first time and in high yield by a reaction under vacuum, entry 1, whereas the known F-type material2 was obtained with hydrogen, entry 2. The ratio VF-type/VM-type ≈ 2.0 (following ZF/ZM ) 2) suggests divalent Eu in both. The Eu-Sb distances, 3.2-3.4 Å, are also consistent with divalent Eu (see Tables 2 and 3); see also the comparison of distance parameters for the Sm systems above. The effective molar
moment of the compound, µeff ) 19.1(3) BM (∼8.55 µB per Eu), also corroborates this assertion. Figure 3 shows the Curie-Weiss plots for this, and the analogous Eu5Bi3, for which µeff ) 16.9(2) BM (∼ 7.57 µB per Eu; Θ ∼ 101 K). These two compare with the free ion value for Eu2+ of 7.94 µB. Yb-Sb. Because the discovery of the supposed orthorhombic β-Yb5Sb3 (Y-type) structure occurred in this system,44 we gave special attention to the postulated M-Y phase transition put forth then and in a recent phase diagram report33 to explain the coexistence of the two phases. Our early unpublished studies were not able to uncover a reasonable composition or temperature basis for the supposed transition, and a considerable variation of lattice constants for different syntheses of the “Y” phase was also observed.45 Higher yields of the Y phase with additional Yb also led to the earlier idea that Yb5Sb2 was the more likely stoichiometry for the orthorhombic phase.32 Experiments under minimum hydrogen concentrations, dv, gave the hexagonal structure as the main phase, entry 1, as did both the loaded composition Yb5.8Sb3 under a dv condition, entry 2, and when cleaned Yb metal was used for an sc reaction, entry 3. Cell dimensions of the hexagonal phase are similar to those we reported earlier,16 whereas those parameters in the first two reports32,33 are less than anything found here, suggesting11 other light atom impurities (C, N, etc.) may have been present as well. As indicated earlier,2 products ranging from mixtures of M and F to single-phase F type were observed for reactions involving hydrogen, and reversibility of the M f F and F f M transitions were demonstrated as well. The structure of a crystal from the Yb-rich reaction, entry 2, refined to Yb5.00(2)Sb3 (Tables 2 and 3) and another from the entry 1 reaction gave a substantially identical solution. Interatomic Yb-Sb distances of 3.09-3.28 Å are close to the sum of the crystal radii for Yb2+ and Sb3-, ∼3.20 Å, according to an anion radius of 2.04 Å deduced from a number of trivalent REPn (NaCl) phases together with the cation crystal radius.37 In addition, all of the distances in
(43) Abdusalyamova, M. N.; Abulkhaev, V. D.; Levitin, R. Z.; Markosyan, A. S.; Popov, Y. F.; Yumaguzhin, R. Y. SoV. Phys. Solid State (Engl. Transl.) 1984, 26, 343.
(44) Brunton, G. D.; Steinfink, H. Inorg. Chem. 1971, 10, 2301. (45) Hurng, W.-M. Ph.D. Dissertation, Iowa State University, Ames, IA, 1988.
Figure 3. Temperature dependencies of molar susceptibilities (χ) and of inverse susceptibilities at 3 T for Eu5Sb3 (9, 0) and Eu5Bi3 (2, ∆).
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the structure of Ca5Sb3 are quite similar to those of the Yb analogue (Table 3), appropriate to the similar crystal radii for the two dipositive cations.37 Magnetic susceptibility measurements on the entries 1 and 2 samples gave small moments, 0.81 and 0.77 µB respectively (vs 4.54 µB for Yb3+), and somewhat negative Weiss constants. These may reflect either small amounts of YbIII in these phases or characteristics of the small amounts of Yb16Sb11 side products. Bismuthides. The driving force for Ca5Bi3 and Yb5Bi3 to pick up hydrogen and readily transform into Y-type phases is remarkably large.2 For this reason, preparation of these M-type materials in high yields requires additional experimental work. Ca-Bi. Among the F-type phases, F-Ca5Bi3H shows the widest range of lattice parameter variations, with a volume variation of ∼1.05%.2 It is in parallel the most stable with respect to hydrogen loss, so much so that the compound was initially thought to be a binary example. dv conditions that work well for other systems afforded only 5% of the hexagonal phase, entry 1, and an sc reaction with cleaned Ca gave only about the same amount of the M material, entry 2. Repetition of reaction 2 in presence of about 10 times as much of a reputedly good hydrogen getter, dehydrogenated Y chips in a second Ta container inside the same silica jacket, led to only a modest 25% increment of the desired phase (entry 3). To retain most of the M phase, it was necessary to quench the high-temperature dv reaction, entry 4, thereby evidently avoiding a gettering of the vacuum system on slow cooling. Lattice dimensions of all M-type materials were substantially equivalent, and the hydrogen solubility therein must be small, giving no more than ∼0.06% in lattice contraction. The phase is reported for first time. Sr-Bi. The M-type phase was prepared in high yield from reactions either under vacuum or from cleaned Sr, entries 1 and 2, respectively. Products with the F-type structure were observed only if hydrogen was purposely loaded, entry 3. There is a measurable concentration (lattice constant) range in M before the transformation, ∆V ∼ 0.3%, the relative silica tube contribution alone being small enough to prevent most F-phase production, entry 2 Ba-Bi. The Ba5Bi3Hx compositions crystallize only in the M structure type. Accordingly, the overall cell volume variation with hydrogen content is almost the largest found, about 2.8%, comparable to that for Yb5As3. Lattice parameters stated to be those of the binary in the literature are grossly different, and all clearly correspond to ternary hydride solutions. Sm-Bi. The reaction under vacuum conditions led to a mixture of M and anti-Th3P4 types. On the other hand, reactions with excess hydrogen gave the known F phase.2 The c/a ratio, the cell volume ratio for the F versus M forms, 2.43, and the appreciable difference between cell volumes of the Sm versus Sr bismuthides (contrary to their very similar divalent crystal radii) all indicate that Sm in the hexagonal phase is probably trivalent, as seems to be true for the antimonide as well. Eu-Bi. Single-phase hexagonal products were obtained under dv conditions, entry 1, the first report of this phase.
Leon-Escamilla and Corbett
The F-type phase is obtained when hydrogen is purposely included.2 The cell volume ratio between F and M forms, ∼2.0, indicates that Eu is divalent in both compounds. This is in agreement with the Curie-Weiss behavior (Figure 3) with µeff ) 19.1(3) BM (∼8.56 µB per Eu ion) and with the Eu-Bi distances calculated from the structural refinement (Table 3). Yb-Bi. As with Ca5Bi3(H), the Yb-Bi system presented some difficulties in the preparation of the binary 5-3 compound. Both examples represent the favorable combinations of small cation and large anion and thus the highest stability for the hydride.2 It again proved necessary to quench a dv sample from high temperature in order to retain a majority of the hexagonal phase, entry 2. The cell volume ratio F/M indicates divalent Yb in both structures, and the M-type cell volume is also close to that of the calcium compound, as are their cation crystal radii.37 This is the first report of the hexagonal 5-3 phase. Although magnetic susceptibilities of pnictides of the rareearth elements afford some useful characterizations of valence states, as demonstrated above, such data for the M-structured binary alkaline-earth-metal (Ca-Ba) antimonides and bismuthides as well as their hydrides have generally provided less clear and simple answers.39 In simple views, the binaries should be metallic with one extra conduction electron per formula, whereas the true monohydrides should be diamagnetic semiconducting Zintl phases, but these assessments are likely too simple. Subsequent theoretical analyses of bonding in related systems by better ab initio methods have shown that mixing of d states on the alkaline-earth-metal (Ae) cations with valence and conduction band states deriving mainly from the anions may be appreciable, and ideal closed anion-based configurations may not exist.22,46 This is particularly the case with idealized Bi3- ions in the stoichiometric (electron-poor) Ae4Bi3 phases with Ae ) Sr or Ba.22 Accordingly, observed magnetic susceptibilities for the present M-type Ae5(Sb,Bi)3 compounds and their hydrides are generally small, positive, and not entirely temperature independent.39 In contrast, the arsenic-deficient compound Ba4As2.6 is a semiconducting valence compound (ideally Ba4As2.67),22 and in parallel, Ca5As3 is better behaved and exhibits a temperature-independent Pauli-like susceptibility, whereas its hydride is diamagnetic, χ ∼ +0.9 and -0.6 emu/mol, respectively, over ∼100-300 K. Cell dimensions of the binary M-type structures plotted against the cation for each compound studied, Figure 4, show the expected linear relationships. Irregular data for two Sm compounds are omitted because of the trivalency of that cation. The ranges of hydrogen content, as measured by percent cell volume contractions for both the M and the F structure types, are summarized in Table 4. These are found to decrease, first according to increasing sizes of the cation, which are most responsible for hydrogen binding, and second, more slowly with increasing anion size, but these trends are dominated by whether there is a competitive equilibrium between M- and F-structured phases or not. Thus (46) Mudring, A.-V.; Corbett, J. D. J. Am. Chem. Soc. 2004, 126, 5277.
Hydrogen in Polar Intermetallics
Chem. Mater., Vol. 18, No. 20, 2006 4791
Figure 5. Scheme of reactions to prepare A5Pn3Hx materials, A ) Ca, Sr, Ba, Sm, Eu, Yb; Pn ) As, Sb, Bi. Reaction conditions; sc ) sealed fused silica container; [+H2] ) addition of binary hydride; dv ) dynamic vacuum; [-H2] ) removal of hydrogen in vacuum; sc-lta ) sealed container with long time annealing.
Figure 4. Cell dimensions of binary alkaline-earth and divalent rare-earthmetal pnictide phases AII5Pn3 (A ) Ca, Sr, Ba, Eu, Yb; Pn ) As (]), Sb (4), Bi (O)). Solid symbols are for the a axis; open symbols, c axis; and blue, red, and green represent As, Sb, and Bi, respectively. (Data for trivalent Sm compounds are omitted.) Table 4. Calculated Cell Volume Decrements for A5Pn3Hx Phases (%) Pn As
A Ca Sr Ba Sm Eu Yb
Ma struct. type 2.12 0.73 smb 2.9
Sb Fa struct. type
Bi
M struct. type
F struct. type
M struct. type
F struct. type
∼0.0 0.3 2.42 0.74 Mc 0.25
0.48 0.22
∼0.06 0.34 2.8 Mc Mc sm
1.05 0.28
0.20 0.50
0.21 0.56
a M ) Mn Si -type structure; F ) Ca Sb F-type structure. Data for F-type 5 3 5 3 phases from ref 2. b sm ) small,
