Article pubs.acs.org/IC
Structural Transformation in Inverse-Perovskite REPt3B (RE = Sm and Gd−Tm) Associated with Large Volume Reduction Sudipta Mondal,† Chandan Mazumdar,*,† Rajarao Ranganathan,† and Maxim Avdeev‡,§ †
Condensed Matter Physics Division, Saha Institute of Nuclear Physics (SINP), 1/AF Bidhannagar, Kolkata 700064, India Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia § School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia ‡
ABSTRACT: In this work, we report the structural phase transformation of tetragonal inverse-perovskite REPt3B (RE = Sm, and Gd−Tm) compounds to cubic perovskite structure, with a large volume reduction of about 9% (reduction of the c axis, ∼17%; increase in the a axis, ∼5%). The structural stability of the cubic phase, however, could only be maintained by lowering the lattice parameter of the off-stoichiometric REPt3Bx (x < 1), formed in the process of annealing. The combined effect of phase transformation and stoichiometric defects is argued to be responsible for the observed volume collapse. Unexpectedly, the application of a large hydrostatic pressure of ∼20 GPa does not have any significant effect on the crystal structure. Neutron diffraction studies and heat capacity measurements unambiguously confirm different magnetic transition temperatures in the tetragonal and cubic phases. The different physical properties of these two phases demonstrate the interrelationship between the crystal chemistry and the physics of the system. The synthetic route to cubic REPt3Bx identified in this work may be utilized to prepare new ternary rare-earth intermetallics in a cubic perovksite form, which was previously found to facilitate unconventional superconductivity.
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INTRODUCTION Many compounds, be it crystalline, glassy/amorphous,1 or liquid system,2,3 are known to have the ability to adopt multiple crystal structures, which controls the physical and chemical properties of those corresponding systems. Such phase transitions normally take place under the application of pressure, temperature, strain, electrical and magnetic field, variation of synthesis procedure, particle size, etc. Structural transitions can be either isomorphic in character, where the space group remains conserved, or polymorphic, where the compound changes its lattice symmetry because of such a transformation. Although most often the structural transitions are not associated with an appreciable change in the crystal packing density, in a very few special cases, it can be quite large. Polymorphic transformation generally favors such a large change in volume due to the rearrangement of atomic coordinations inside the crystal. Largest reversible volume collapse associated with a polymorphic structural transition has been reported in insulating MnS2 (∼22%),4 while among oxide perovskites, BiCoO3,5,6 PbVO3,7,8 and PbCrO39,10 exhibit ∼13%, ∼10.6%, and ∼10% volume reductions, respectively. In all of these systems, the structural transitions are driven by the application of temperature or pressure, while the chemical compositions remain conserved. However, it was often found that even an introduction of partial vacancies in a particular crystal structure may not change the lattice parameters significantly. Extreme cases, for example, are CeRh3B (perovskite ABX3-type, space group Pm3̅m) and CeRh3 (AuCu3-type, © 2017 American Chemical Society
i.e., perovskite structure with a vacant body-center position, space group Pm3̅m), where lattice parameters are not found to be appreciably different.11 A similar behavior holds true for binary REPd3 and ternary inverse-perovskite (or antiperovskite) REPd3Bx (x < 1) series of compounds.12 The latter ternary antiperovskite series is not known to form in a single phase with full stoichiometry of boron. Except for REPd3Bx, other rare-earth intermetallic antiperovskite compounds, viz., RERu3C, RERh3C, and RERh3B, form with full stoichiometry. The system also forms in full stoichiometry when the Pd 4d element is replaced by the Pt 5d element. In this case, however, to accommodate the larger-sized platinum, the crystal structure changes from cubic perovskite to the tetragonal CePt3B-type (space group: P4mm).13 This resulted in considerable elongation of the c axis (c/a ∼ 1.26), by accommodating boron in a slightly off-center position forming aBPt5 pyramidal. On the other hand, in an ideal cubic perovskite, boron remains in the center of the BPt6 octahedron. In comparison to the ideal perovskite cubic structure, irrespective of the boron concentration, the lattice parameter a of the tetragonal structure is slightly reduced, whereas the lattice parameter c is considerably enhanced (∼15−20%), resulting in substantial volume expansion. Removal of the inversion symmetry thus allows a bigger space for the lighter atom. As a consequence, the boron site (Wyckoff position: 1b) found to be fully occupied in Received: May 5, 2017 Published: July 5, 2017 8446
DOI: 10.1021/acs.inorgchem.7b01131 Inorg. Chem. 2017, 56, 8446−8453
Article
Inorganic Chemistry
Figure 1. XRD analysis of REPt3B compounds. (A1−G3) Rietveld refinement of the powder XRD patterns at room temperature and calculated Bragg positions for space groups P4mm (T-panel) and Pm3̅m (C-panel) of as-cast and annealed (at 850 and 1200 °C) REPt3B (RE = Sm and Gd− Tm) compounds.
tetragonal CePt3B in contrast to cubic CePd3Bx,12 CePd3Six,14 CePd3Gax15 (x < 1), etc. The tetragonal structure remains stable even if boron is replaced by the larger-sized silicon in stoichiometric CePt3Si, whereas the solubility limit of silicon in cubic CePd3Six is only 30%.14 Such a structural rearrangement appears to be rather sensitive to the lattice volume because only the compounds having larger tetragonal unit-cell volumes are reported to form with lighter RE elements, viz., REPt3B (RE = La−Nd)16 and REPt3Si (RE = La−Gd).17 The heavier rareearth analogues of REPt3B (RE = Eu−Lu) compounds, which are not reported so far in the tetragonal structure, plausibly because of their reduced lattice volume, are expected to exert larger pressure on the boron site and place it at the body-center position. If there is not enough free space for the same at the body-center position, one may still expect it to form in the cubic perovskite structure, with a limited solubility of boron, as in the case of CePd3Bx (x < 1). In this work, we report the successful synthesis for the first time of heavier ternary REPt3B (RE = Sm and Gd−Tm) compounds. We also show that the tetragonal phase is metastable in character because, upon
annealing at high temperature, it rejects boron partially and stabilizes in the ternary cubic perovskite REPt3Bx (x < 1) phase. This transformation is associated with a large volume reduction of ∼9%, with a reduction of the c axis by ∼17% and an increase in the a axis by ∼5%. Notwithstanding the slight variation of the boron concentration, the different physical properties of these two phases reveal the role of the crystal chemistry on the physics of similar systems. For example, as a consequence of our work, if CePt3Six (x ≤ 1) could be synthesized in centrosymmetric cubic form, one could understand the origin of unconventional superconductivity reported in its noncentrosymmetric tetragonal structure.17−19
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EXPERIMENTAL SECTION
The polycrystalline REPt3B (RE = Sm and Gd−Tm) compounds were synthesized for the first time using standard arc melting techniques under a flowing argon atmosphere. The ingots were flipped over and remelted five to six times to ensure homogeneity. The as-cast ingots were then wrapped in tantalum foil, sealed in a quartz tube under vacuum, and annealed at 850 and 1200 °C for 1 week. The respective 8447
DOI: 10.1021/acs.inorgchem.7b01131 Inorg. Chem. 2017, 56, 8446−8453
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Inorganic Chemistry
Table 1. Crystallographic Parameters from Rietveld Refinement of XRD Data of REPt3B (RE = Sm and Gd−Tm) Compounds cell parameters compound SmPt3B
heat treatment
DyPt3B
as-cast annealed 850 °C annealed 1200 °C as-cast annealed 850 °C annealed 1200 °C as-cast annealed 850 °C annealed 1200 °C as-cast
HoPt3B
annealed at 850 °C annealed at 1200 °C as-cast
ErPt3B
annealed at 850 °C annealed at 1200 °C as-cast
TmPt3B
annealed at 850 °C annealed at 1200 °C as-cast
GdPt3B
TbPt3B
at at
at at
at at
annealed at 850 °C annealed 1200 °C
atomic coordinates
phase
a(Å)
c(Å)
V(Å3)
z(Pt1)
z(Pt2)
Rp
RBragg
Rf
tetragonal (100%) tetragonal (98%) cubic (2%) tetragonal (47%) cubic (53%) tetragonal (100%) tetragonal (91%) cubic (9%) tetragonal (60%) cubic (40%) tetragonal (100%) tetragonal (82%) cubic (18%) tetragonal (28%) cubic (72%) tetragonal (87%) cubic (13%) tetragonal (74%) cubic (26%) tetragonal (14%) cubic (86%) tetragonal (91%) cubic (9%) tetragonal (78%) cubic (22%) cubic (100%)
3.9745(2) 3.9738(1) 4.1637(7) 3.9745(2) 4.1717(3) 3.9655(2) 3.9638(1) 4.1529(3) 3.9744(3) 4.1531(3) 3.9517(3) 3.9547(1) 4.1428(2) 3.9655(3) 4.1397(3) 3.9502(2) 4.1219(5) 3.9477(1) 4.1358(2) 3.9598(3) 4.1309(2) 3.9433(2) 4.1091(7) 3.9421(2) 4.1278(2) 4.1279(1)
5.0208(3) 5.0262(2)
79.3 79.4 72.2 79.4 72.6 78.7 78.7 71.6 79.8 71.6 78.5 78.2 71.1 79.5 70.9 77.9 70 77.8 70.7 79.2 70.5 77.5 69.4 77.5 70.3 70.3
0.5258(13) 0.5115(11)
0.1175(10) 0.1168(10)
21.9 27.8
0.5110(23)
0.1164(21)
25.9
0.5297(20) 0.5110(9)
0.1233(14) 0.1165(8)
38.7 20.8
0.5115(22)
0.1204(20)
23.4
0.5269(13) 0.5163(8)
0.1242(9) 0.1164(7)
22.5 14.8
0.5144(27)
0.1196(25)
17.9
0.5140(12)
0.1181(11)
21.8
0.5142(10)
0.1160(8)
16.2
0.5404(5)
0.1528(3)
17.1
0.5078(10)
0.1158(10)
14.7
0.5077(11)
0.1159(10)
18.4
9.26 11.9 29.4 7.9 8.82 6.96 4.79 6.68 5.52 4.2 7.22 4.73 2.67 8.17 2.83 5.49 8.84 3.18 2.83 4.44 1.31 3.92 3.44 4.75 1.43 7.57
9.39 10.9 24.5 6.79 6.76 10.2 4.28 4.31 5.87 4.36 8.23 4.08 2.80 7.89 3.82 5.94 6.79 2.67 2.3 4.55 1.08 3.72 2.62 4.03 1.4 5.61
tetragonal (92%) cubic (8%) tetragonal (88%) cubic (12%) tetragonal (18%) cubic (72%) tetragonal (89%) cubic (11%) tetragonal (78%) cubic (22%) cubic (100%)
3.9330(1) 4.1032(11) 3.9346(1) 4.1237(3) 3.9333(5) 4.1066(7) 3.9281(3) 4.0836(8) 3.9308(2) 4.1173(4) 4.1003(1)
6.34 5.96 9.37 3.33 5.74 1.73 4.79 5.07 8.55 2.7 12.4
4.68 3.27 5.38 3.82 4.65 1.13 4.93
5.0267(5) 5.0053(4) 5.0084(2) 5.0540(6) 5.0026(4) 5.0012(3) 5.0574(7) 4.9946(4) 4.9952(3) 5.0557(1) 4.9877(4) 4.9892(3)
4.9847(2) 4.9867(3) 4.9868(10) 4.9806(6) 4.9806(4)
annealing temperatures were raised in 6 h, and after 1 week, it was again cooled to room temperature in 6 h. An additional batch of samples was also annealed for a short time, such as for 1 day, to check the effect of the annealing time on the crystal structure. We have preferred slow cooling [i.e., low-temperature (LT) modification] over quenching [i.e., high-temperature (HT) modification] after annealing because the later process is somewhat akin to as-cast materials that may be considered to be quenched from its melting temperature. The structural characterization of the samples was performed by a powder X-ray diffraction (XRD) technique at room temperature using Cu Kα radiation on a Rigaku TTRAX-III powder diffractometer, having a rotating-anode X-ray source at 9 kW. Before the measurements, the diffractometer was calibrated using a standard silicon sample supplied by the manufacturer of the diffractometer. The Rietveld refinements of the powder XRD patterns were performed by using the FullProf program package.20 The full Rietveld refinement of XRD data was performed by keeping all of the atomic displacement parameters equal to zero. The positional parameters and occupancy factor of boron are also kept constant because, with boron being a lighter element (Z = 5), they would have a limited effect on the XRD patterns. The angledispersive XRD patterns at high pressures and room temperature were
77.1 68.7 77.2 70.1 77.1 69.2 76.8 68.3 76.9 69.7 68.9
16.4 0.5078(10)
0.1158(10)
21.8
0.50782(13)
0.1160(13)
24.7
0.5167(62)
0.1304(55)
18.1
0.5175(13)
0.1166(12)
0.5148(11)
0.1148(11)
21.4 6.41 22.5 19.6
5.89 2.54 8.79
obtained at PETRA-III, P02.2 beamline at DESY Photon Science, Hamburg, Germany. The powdered sample was loaded into a diamond-anvil cell with a pressure medium of methanol:ethanol = 4:1 to provide hydrostatic conditions. The pressure was determined by the Ruby fluorescence method. The powder XRD data were collected using monochromatic X-rays (wavelength, λ = 0.289 Å) on an imageplate detector. The two-dimensional XRD images were analyzed using FIT2D21 software, yielding a conventional one-dimensional XRD pattern. The sample-to-detector distance and other geometrical parameters were obtained using a CeO2 standard. The neutron diffraction (ND) experiments were carried out at the powder neutron diffractometer beamline ECHIDNA (ANSTO, Australia), using a wavelength of 2.4395 Å. Multiple separate batches of TbPt311B, ∼3 g of each, were synthesized using a 11B isotope for the ND experiments. Some of these batches were annealed in the same procedure as mentioned above. The sample was held in a cylindrical vanadium can in a cryostat, and ND patterns were recorded in the temperature range of 1.26−300 K. The Rietveld refinement of the powder ND patterns was also carried out by using the FullProf program package.20 The heat capacity measurements were performed using a Quantum Design PPMS system. 8448
DOI: 10.1021/acs.inorgchem.7b01131 Inorg. Chem. 2017, 56, 8446−8453
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Inorganic Chemistry
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RESULTS AND DISCUSSION The experimental, calculated, and difference powder XRD patterns at room temperature of as-cast and annealed (at 850 and 1200 °C) REPt3B compounds are depicted in parts A1− G1, A2−G2, and A3−G3 of Figure 1. The full Rietveld refinements of the XRD patterns of powdered samples taken at room temperature reveal that the as-cast REPt3B (RE = Sm, Gd, and Tb; Figure 1A1−C1) compounds essentially form in a single phase having the CePt3B-type tetragonal crystal structure (space group: P4mm). The lattice parameters for SmPt3B, GdPt3B, and TbPt3B (Table 1) thus obtained follow the usual lanthanide contraction curve, consistent with the decrease of the atomic size of the RE metals with increasing atomic number (Figure 2). The XRD patterns of as-cast samples of the heavier
energy-dispersive analyses of X-ray (EDAX) results suggest that the compounds remain homogeneous on the RE and platinum concentrations (1:3), within the resolution limit of EDAX measurement. Boron, being a lighter element (Z = 5), is insensitive to EDAX. ND results (discussed later) were used to estimate the boron content. The as-cast samples were annealed at 850 °C for 7 days, with the aim of reducing the fraction of the additional phase. However, the room temperature XRD patterns of these annealed REPt3B (RE = Dy−Tm) compounds (Figure 1D2− G2), instead of the expected reduction, rather reveal considerable enhancement (almost doubled) in the intensity of those additional XRD lines belonging to the nontetragonal perovskite phase. The additional lines are also observed in the XRD patterns of annealed SmPt3B, GdPt3B, and TbPt3B (2%, 9%, and 18% of maximum intensity lines, respectively), which, as mentioned earlier, formed in a single phase in as-cast conditions (Figure 1A2−C2). After the samples were annealed at much higher temperature, viz., at 1200 °C for 7 days, the additional line intensities were found to enhance even further, so much so that the XRD peaks associated with the tetragonal phase reduced to less than 50% in most cases (Figure 1A3−G3), while disappearing completely in the cases of HoPt3B and TmPt3B. The XRD peaks for these two compounds could be indexed with the ideal cubic Pm3̅mtype perovskite crystal structure, where the cubic phase has about 9% smaller volume than the respective tetragonal isomers. Such a large volume collapse during irreversible structural changes in intermetallic compounds is rather rarely observed. The readjustment of boron in the body-center position due to annealing at high temperature might result in an unusually large reduction (∼17%) of the c axis, accompanied by a ∼5% increase in the a axis (9% effective reduction in the total volume). Thus, we found that, while the as-cast samples form primarily in the tetragonal structure, annealing at higher temperatures yields the formation of cubic phases at the expense of the tetragonal structure. We also point out here that the XRD patterns of the annealed compounds did not change appreciably with respect to those of the as-cast samples, when the samples were annealed for a shorter period of 1 day. It is interesting to investigate whether the elemental composition of REPt3B remains conserved in the cubic phase because binary REPt3 compounds with RE = Tb−Lu are also known to form in a cubic AuCu3-type structure,22 considered as a perovskite compound with a void in the body-center position. The lighter REPt3 (RE = La−Tb)22,23 form in a cubic C15b type of phase, and only TbPt3 is reported to change its crystal structure upon annealing from a C15b type of phase to a AuCu3-type structure. In our case, the lattice volume per formula unit for a cubic REPt3B perovskite phase was found to be larger than those of their corresponding binary counterparts REPt3 (RE = Tb−Lu) because of the introduction of boron in the body-center position (inset in Figure 2A). A similar enhancement of the lattice parameter due to the incorporation of boron in REPd3 was earlier reported in the REPd3Bx (0 < x < 1)24 series. The enhanced lattice parameter of the ternary phase thus confirms the presence of boron (0 < x ≤ 1) in the bodycenter position in cubic REPt3B compounds. Following this result, we attempted a detailed analysis of the XRD patterns of all of the as-cast and annealed REPt3B (RE = Sm and Gd−Tm) compounds considering both the tetragonal (space group: P4mm) and cubic (space group: Pm3̅m) phases. The XRD patterns of all of the samples, both as-cast (RE =
Figure 2. Variations of the lattice parameters of REPt3B compounds. (A) Cubic lattice parameters. The inset shows the variations of the cubic lattice parameters of C15b-type REPt3 (1), AuCu3-type REPt3 (2), and cubic perovskite-type REPt3B (3) compounds. (B) Tetragonal lattice parameters along the a axis. (C) Tetragonal lattice parameters along the c axis. (D) Tetragonal c/a ratio. (E) Volume of the tetragonal and cubic phases. The data for REPt3B (RE = La−Nd) in B−E and for REPt3 (1 and 2) in the inset of part A are taken from refs 16 and 22, respectively, while the rest are outcomes of this work. The solid lines are guides to the eye.
members of the REPt3B (RE = Dy−Tm) series, however, show the presence of additional lines that are not permitted in the P4mm space group, suggesting the presence of an additional phase of about 10% of the tetragonal volume fraction (Figure 1D1−G1). These additional XRD lines remain in different batches of all of the samples synthesized, although the relative intensity ratios may not always be the same. However, the 8449
DOI: 10.1021/acs.inorgchem.7b01131 Inorg. Chem. 2017, 56, 8446−8453
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Inorganic Chemistry Dy−Tm) and annealed at 850 and 1200 °C (RE = Sm and Gd−Tm), could be indexed well with a two-component model (Figure 1D1−G1,A2−G2,A3−G3). The lattice parameters of all of the systems studied here are presented in Figure 2A−E, while the structural parameters obtained from the Rietveld refinement procedure are given in Table 1. As expected, the heavier members of this series (except ErPt3B) with smaller crystal volume, have higher probability of boron atom getting pushed in the body-center position. Similar results were found in all the different batches of the same compounds measured under same annealing conditions, although a minor variation of relative ratios of the two phases could be noted. Such small difference can be attributed to a small compositional variation, particularly that of boron, that can not be controlled well in arc melting technique. It should be mentioned here that, during the above analysis of the XRD data, the occupancy parameter for the boron atom was held constant, because boron, being a lighter element (Z = 5), is almost invisible to XRD. However, unlike XRD, in the ND measurements, the occupancy of the boron position could be refined through Rietveld analysis. For this objective, ND has been carried out, as an example, for the TbPt3B compound (synthesized using the 11B isotope), in both as-cast and annealed (at 1200 °C) forms, where different batches of TbPt3B synthesized separately were combined together. Parts A and B of Figure 3 represent analysis of the ND patterns of as-
a cubic phase in addition to the presence of two tetragonal phases. The full Rietveld analysis indicates the cubic phase to be of composition TbPt3B0.32 with a volume fraction of ∼50% (Table 2). The two tetragonal phases are found to be TbPt3B0.76 and TbPt3B0.9, having volume fractions of ∼33% and ∼17%, respectively. Because the ND pattern of the sample had been collected by combining together different batches of TbPt3B synthesized separately, it appears that the rates of extrusion of boron from different batches of the sample are different, resulting in two tetragonal phases of TbPt3Bx of slightly different boron concentrations with almost the same volume. It therefore appears that under the annealing conditions the HT tetragonal structure is metastable and transforms into the LT cubic phase with reduced boron content. The structural transition thus takes place for a boron concentration somewhere between 0.75 < x < 0.32. It may be noted here that the hypothetical stoichiometric cubic perovskite would have larger lattice parameters, which may affect the structural stability of this system. In order to form in a stable cubic phase, the boron content is partially reduced during phase transformation, thereby reducing the lattice parameters. This boron deficiency is also in accordance with the earlier report25 that the structural stability of the intermetallic cubic perovskite system is governed by the valence-electron concentration per unit cell, whose value should remain in the range of 31−34.3. Accordingly, cubic REPt3Bx could form for the boron concentration not exceeding 0.43, similar to those reported earlier in many members of the REPd3Bx12 series. It may thus be safely inferred that, from the structural aspect, the ND result agrees well with the XRD result. However, because ND analysis suggests the partial occupancy of boron in the cubic TbPt3Bx system, a large change in the volume was estimated between the tetragonal and cubic phases and therefore cannot be attributed to the phase transformation only. Rather the volume collapse should be considered as a combined effect of both the structural transition and the compositional effect. An attempt was also made to force the boron to the bodycenter position by exerting large external pressure, instead of annealing at high temperature. Among the oxide perovskite compounds, PbTiO326 and BaTiO327 with c/a ∼ 1 undergo reversible structural transformation with a negligible volume change, whereas PbVO3, which is structurally very similar (the same P4mm space group and similar high c/a ratio) to the tetragonal REPt3B phase, was earlier reported to exhibit a structural transition from the tetragonal to cubic phase at room temperature associated with a volume change of 10.6% under the application of a pressure of 2.7 GPa.7 To check this possibility, we have studied the effect of pressure on the structure of two representatives, viz., as-cast GdPt3B and HoPt3B, containing 100% and 91% tetragonal phases, respectively, at room temperature. Surprisingly, none of these two compounds exhibit any structural phase changes even up to 20 GPa pressure (Figure 4). The GdPt3B compound maintains
Figure 3. ND patterns of TbPt3B. (A and B) Rietveld refinement of the powder ND patterns at room temperature and calculated Bragg positions for space groups P4mm (T-panel) and Pm3̅m (C-panel) of the as-cast and annealed (at 1200 °C) TbPt3B compounds.
cast and annealed (at 1200 °C) TbPt3B compounds at room temperature. In the as-cast TbPt3B compound, the boron site was found to be fully occupied in the tetragonal phase (Table 2). The pattern of annealed TbPt3B could be indexed well with
Table 2. Crystallographic Parameters from the Rietveld Refinement of ND Data of TbPt3B Compounds cell parameters
atomic coordinates
compound
phase
a(Å)
c(Å)
V(Å3)
z(Pt1)
z(Pt2)
z(B)
Rp
RBragg
Rf
as-cast TbPt3B annealed TbPt3B
T: TbPt3B T1: TbPt3B0.76(2) T2: TbPt3B0.89(4) C: TbPt3B0.32(0)
3.9549(2) 3.9705(2) 3.9849(2) 4.1507(2)
5.0010(2) 5.0344(3) 5.0768(4)
78.2 79.3 80.6 71.5
0.5151(9) 0.5117(17) 0.5117(17)
0.1219(8) 0.1271(17) 0.1271(17)
0.7058(11) 0.7070(25) 0.6970(25)
12 10.8
4.74 5.07 10.1 1.17
4.04 4.11 7.21 1.65
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DOI: 10.1021/acs.inorgchem.7b01131 Inorg. Chem. 2017, 56, 8446−8453
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Inorganic Chemistry
Figure 4. Effect of the hydrostatic pressure on the REPt3B compounds. The experimental powder XRD patterns and the calculated Bragg positions for space groups P4mm (T-panel) and Pm3̅m (C-panel) of as-cast GdPt3B and HoPt3B compounds measured at the lowest and highest pressure available. The XRD patterns are taken using the wavelength of 0.289 Å but later calibrated for a wavelength of 1.5418 Å. The insets show the lattice parameter variations with pressure. The solid lines are guides to the eye.
its tetragonal crystal structure, while in the case of HoPt3B, the relative intensities of the reflection peaks from the cubic structure do not enhance with the application of higher pressures either. This is consistent with our earlier observation that the said structural transformation is associated with the introduction of boron vacancies, which is not possible with the application of pressure (at least up to 20 GPa). Variations of the lattice parameters with the application of pressure for both GdPt3B and HoPt3B, are shown in the insets of Figure 4. The average rate of change of the c parameter, dc/dp, for GdPt3B was estimated to be around 0.002 Å GPa−1. The physical properties of these two phases, viz., tetragonal and cubic, are also found to be quite different. The insets (2) and (4) of Figure 5 present, as an example, the heat capacity results of two different samples of TbPt3B, in as-cast and annealed forms at 1200 °C, respectively. The as-cast compound exhibits a single λ-like peak at around ∼16.5 K in the heat capacity data, representing the bulk magnetic ordering temperature of the tetragonal phase of TbPt3B. The heat capacity data of the annealed TbPt3Bx, on the other hand, exhibit two such peaks, at ∼5 and ∼17 K, with the lower one belonging to the cubic phase. The transition temperatures are also confirmed by a LT ND experiment [insets (1) and (3) of Figure 5]. Analysis of the ND data of the annealed sample, considering a two-phase model, suggests the magnetic structures to be G-type antiferromagnetic with the wave vector k = (1/2 , 1/2 , 1/2) for both phases, while the ordered state magnetic moment of Tb ions at 1.2 K are ∼7 and ∼5.5 μB/f.u. for the tetragonal and cubic phases, respectively. The magnetic ordering temperature of the cubic TbPt3Bx phase is found to be lower than that of both the C15b- and AuCu3-type phases of its binary analogue (TbPt3), thus confirming the presence of boron in the body-center position.
Figure 5. ND patterns of as-cast and annealed (at 1200 °C) TbPt3B compounds. (A−C) Diffraction data taken at 25 K (paramagnetic state) and 1.2 K (magnetic state) and their difference for the as-cast TbPt3B compound. (D−I) Diffraction data taken at 25 K (paramagnetic state), 8 K (paramagnetic cubic and magnetic tetragonal), and 1.2 K (magnetic state) and their differences for the annealed TbPt3B compound. The insets (1) and (2) show variations of the intensity with temperature (left side) and the temperature dependence of the specific heat of the as-cast TbPt3B compound (right side) with TN(T) = 16.5 K. The insets (3) and (4) show variations of the intensity with temperature (left side) and the temperature dependence of the specific heat of the annealed TbPt3B compound (right side) with TN(C) = 5 K and TN(T) = 17 K.
Tm) may not be the ground-state configuration in this structure because annealing introduces a vacancy-induced structural phase transition to the ideal cubic perovskite structure, associated with a considerably large reduction of the volume of ∼9%. In fact, the reduction of the c axis (∼17%) is significantly higher. Among the intermetallic systems, only a very few, viz., ternary REIr2Si2,28 RENi2As229 (tetragonal CaBe2Ge2 to ThCr2Si2 types),30−32 and TbPdAl33 (hexagonal ZrNiAl to orthorhombic TiNiSi type)34 and binary Gd2Fe17 (rhombohedral to hexagonal)35 and TbPt3 (cubic C15b to AuCu3 type)22 are known to exhibit irreversible transformation upon annealing at high temperatures. It may, however, be noted that, unlike the REPt3Bx system studied here, these compounds
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CONCLUSION In conclusion, we report the formation of new compounds of heavier REPt3B (RE = Sm and Gd−Tm) in two different crystal structures: a metastable HT tetragonal phase (REPt3B) and a stable LT cubic phase (REPt3Bx) (0 < x < 1). The large tetragonal distortion in the as-cast REPt3B (RE = Sm and Gd− 8451
DOI: 10.1021/acs.inorgchem.7b01131 Inorg. Chem. 2017, 56, 8446−8453
Inorganic Chemistry
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show neither compositional variation nor any significant volume change. Our result of the formation of two phases of the CePt3B type of compounds may have a significant effect on understanding the physics of similar systems. For example, it was earlier reported that CePt3Si, forming in the same CePt3B type of tetragonal structure, is the first noncentrosymmetric system to exhibit unconventional superconductivity (TC = 0.75 K) below its antiferromagnetic transition at TN = 2.2 K.17 It was later found that many different batches of this compound also exhibit a second superconducting transition at TC ∼ 0.46 K.18 The unconventional nature of the superconductivity was proposed because of the presence of both the spin-singlet and -triplet states, although the noncentrosymmetric nature of the crystal structure generally favors the spin-singlet state only.17 However, this proposal has been contested, where it is suggested that the HT superconducting transition is due to the presence of disordered domains that exhibit conventional swave BCS superconductivity.36 Because we have shown that cubic phases of the REPt3Bx (x < 1) system have much lower magnetic transition temperatures, the superconducting transition temperature in the cubic phase is expected to be larger in comparison to that found in its tetragonal phase and, consequently, one may wonder whether the superconducting transition at 0.75 K is due to the presence of disordered domains36 or a trace amount of the cubic CePt3Six (x < 1) phase. Because our work suggests a way to successfully synthesize the cubic phases of ternary rare-earth-based REPt3Bx (x < 1) borides (and possibly silicides as well), further studies of the properties of tetragonal and cubic counterparts may help to understand the origin of unconventional superconductivity in noncentrosymmetric CePt3Si systems.
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ASSOCIATED CONTENT
Accession Codes
CCDC 1550418−1550450 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chandan Mazumdar: 0000-0001-9143-5349 Maxim Avdeev: 0000-0003-2366-5809 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported through the CMPID-DAE project at SINP. We thank Dr. Konstantin Glazyrin for his help during the high-pressure experiments at PETRA-III, P02.2 beamline, at DESY Photon Science, Hamburg, Germany. This high-pressure experiment was supported through the DST project (Grant I20130217), Government of India. We also thank Dr. Robert Robinson for his support and encouragement for the ND experiments at beamline ECHIDNA at Bragg Institute Neutron Beam Facility, ANSTO, Australia. 8452
DOI: 10.1021/acs.inorgchem.7b01131 Inorg. Chem. 2017, 56, 8446−8453
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