CRYSTAL GROWTH & DESIGN
Effect of K T Rb Substitution on Structure and Phase Transition in Mixed KxRb1-xPb2Br5 Crystals
2009 VOL. 9, NO. 5 2248–2251
Ludmila I. Isaenko,*,† Alexandr A. Merkulov,† Svetlana V. Melnikova,‡ Viktor M. Pashkov,† and Alexandra Yu. Tarasova† Institute of Geology and Mineralogy, SB RAS, Prospect Ak. Koptyuga, 3, NoVosibirsk, Russian Federation, 630090, Kirensky Institute of Physics, SB RAS, Academgorodok, Krasnoyarsk, Russian Federation, 660036 ReceiVed September 11, 2008; ReVised Manuscript ReceiVed January 14, 2009
ABSTRACT: New crystals of the set KxRb1-xPb2Br5 with 0 e x e 1, both pure and Er3+ doped, were grown using the Bridgman technique. The mechanism of structural transformation for a potassium to rubidium substitution with the symmetry change from monoclinic P21/c to tetragonal I4/mcm (at x ) 0.35) is suggested. The origin of rare earth atoms incorporation into the crystal structure of new compounds is explained. Polarization-optical observations reveal a dependence of the temperature of phase transition in mixed crystals on their composition.
1. Introduction MPb2Br5 with M ) K or Rb has attracted attention as a promising laser matrix for obtaining coherent radiation on radiative transitions in rare earth ions (REI) in the mid-IR.1 These crystals are characterized by relatively high chemical and radiative stability. Low energy of the phonon spectrum with hω ∼ 140 cm-1 provides low nonradiative relaxation rates and a high quantum yield for transitions in UV, visible, near midIR spectral regions. These features of RE3+:KPb2Br5 crystals create real possibilities for highly efficient laser action in midIR and make them promising active media for direct laser-diodepumped solid-state lasers emitting in the near- and mid-IR spectral range up to 10 µm. Use as active media requires high optical quality of the crystals. It was found that KPb2Br5 (KPB) crystals undergo a ferroelastic first-order phase transition at 519.5/518.5 K in the heating/cooling regimes, respectively. The transition is accompanied by mmm - 2/m (P21/c) symmetry changes.2 Phase transition below melting temperature (Tmelting(KPB) ) 655 K) causes formation of twin structure. Besides, the crystal is characterized by a high anisotropy in coefficients of the thermal expansion along crystallographic directions, which also stimulates defect formation.3 As a consequence, the light scattering centers appear and the efficiency of stimulated emission generation is lowered due to optical losses. On the other hand, the RbPb2Br5 (RPB) crystal has tetragonal I4/mcm symmetry,4 has no phase transitions up to the melting temperature, and crystallizes without formation of typical extended defects.2 The REI incorporation available dopant concentration in RPB was found to be an order lower than that in KPB. The need to create an optical crystal with a satisfactory level of REI doping resulted in a study of optical quality of mixed KxRb1-xPb2Br5 crystals as well as of the mechanism of structural change for Rb f K substitution.
2. Crystal Growth Synthesis of KxRb1-xPb2Br5 compounds with 0 e x e 1 was performed using high purity bromide salts. The starting high * Corresponding author. E-mail:
[email protected]. Phone/Fax: 7-383-33338-43. † Institute of Geology and Mineralogy. ‡ Kirensky Institute of Physics.
purity reagents PbBr2, KBr, and RbBr, 99.999%, were additionally purified by repeated directed crystallization with prior removal of dirty parts. REBr3 was synthesized from RE oxides, 99.99%, with further distillation. KxRb1-xPb2Br5 single crystals were grown using the Bridgman technique in soldered ampules in halogen atmosphere.5 In order to prevent compound decomposition, the pressure inside the ampule exceeded the atmospheric pressure. As it follows from phase diagrams, KPB and RPB melt congruently at 655 K.5 Linear temperature gradient in a growth zone of the furnace was ∼ 20 K/cm, and the rate of the ampule moving into the cold zone was 2-4 mm/day. One of the starting reagents (PbBr2) hydrolyzes easily when in contact with air. Therefore, a refinement of moisture and oxygen-containing impurities is necessary during synthesis and preparatory operations. Moreover, if material contains water molecules or other impurities, PbBr2 decomposition occurs. It is also important to prevent any contact between purified material and air (i.e., oxygen and water) in both handling operations of starting materials and single-crystal growth process. Because RE bromides are sensitive to oxygen and moisture, it is imperative that both O2 and H2O be strictly excluded from the system. In this case the REI doped KxRb1-xPb2Br5 crystals are very stable and the high quality optical elements from them can be kept in air up to 1 year without considerable destruction (over 2 years in exicator). KxRb1-xPb2Br5 samples with x ) 0, 0.15, 0.3, 0.4, 0.5, 0.7, and 1.0 were grown. Besides, Er doped mixed crystals were obtained. The samples composition and the REI concentration were determined using a microprobe GXL - 8100 device. The RE segregation coefficient was found to depend considerably on crystal composition. When introducing 3 at. % Er into the melt, a 0.04 to 0.22 increase of the segregation coefficient was obtained if x changed from 0 to 0.6.
3. Structural Analysis of Crystals KxRb1-xPb2Br5 with 0 exe1 The X-ray diffraction data were collected at room temperature on a Bruker D8 diffractometer with Cu KR (λ ) 1.5418 Å) radiation. Parameters of the crystallographic cells were obtained modeling the powder data with a PCW 2.4 (PowderCell for Windows, Version 2.4, W. Kraus & G. Nolze) software. They are given in Table 1. For structural analysis, single-crystal data
10.1021/cg8010162 CCC: $40.75 2009 American Chemical Society Published on Web 03/06/2009
Mixed KxRb1-xPb2Br5 Crystals
Crystal Growth & Design, Vol. 9, No. 5, 2009 2249
Table 1. The Lattice Parameters Obtained from the X-ray Structural Analysis of the KxRb1-xPb2Br5 Powders X
a
b
c
0 0.15 0.3 0.4 0.5 0.7 1
8.437 8.433 8.416 9.332 9.314 9.295 9.262
8.437 8.433 8.416 8.41 8.412 8.401 8.363
14.572 14.51 14.474 13.053 13.053 13.045 13.016
β
V
90.22 90.1 90.24 89.98
1037.3 1032.1 1025.2 1024.4 1022.7 1018.6 1008.3
for KPB and RPB were used, which are given in refs 6 and,7 respectively. The ionic radii were calculated from the bond lengths. At first, the average of dense anion-anion distances of closed packing of cation polyhedron were determined. The Br- ionic radius was taken to be equal to the half of this value. Afterward, the average length of the cation-anion bond was calculated. Then, the cation radius was determined by subtracting the anionic radius from this value. In the tetragonal RPB unit cell the coordination RbBr10 polyhedron is a bicapped square antiprism. The polyhedron includes 24 short Br--Br- distances 3.72-4.47 Å long with 4.11 Å average length. Hence we can conclude that the Brradius for this polyhedron is 2.05 Å. The average length of the Rb+-Br- bond is 3.59 Å; therefore, the Rb+ radius is equal to 1.54 Å. The PbBr8 coordination polyhedron is a bicapped trigonal prism. The average length of 17 dense Br--Brdistances is 3.95 Å, which means that the Br- anion radius is 1.98 Å. Subtraction of this value from the average length of the Pb2+-Br- bond results in 1.22 Å Pb2+ radius. The obtained values agree well with the effective Shannon ionic radii for different coordination numbers: (Rb+(X) - 1.66 Å and Pb2+(VIII) - 1.29 Å).8 As expected, the Br- radius in the Rb polyhedron with coordination number N ) 10 is larger than that in the Pb polyhedron with N ) 8, and both these values exceed the tabular Br radius for N ) 6 (Br-(VI) - 1.96 Å). The cationic radii of Rb+ and Pb2+ are smaller than the tabular values because of the covalent input to ionic bonds in bromides is increased in comparison with the oxygen compounds. The structure of monoclinic KPb2Br5 crystal is described in ref 6. The KBr9 coordination polyhedron is a monocapped square antiprism. Values of 2.08 Å and 1.42 Å were obtained for Br- anionic and K+ cationic radii, respectively. The Pb2+ cations occupy two structurally different sites in the monoclinic crystal. The Pb(1) structural position is analogous to that of K+, although it is characterized by a smaller N value. The Pb(1)Br8 polyhedron is a distorted capped tetragonal antiprism. Distortion results in compressing of one square of the antiprism: This square can be described by two triangles with a common side. The anionic radius is 1.98 Å, and the cationic one is 1.22 Å. The obtained values are equal to two analogous values for the PbBr8 in RbPb2Br5. It confirms the accuracy of the ionic radii calculations. The coordination Pb(2)Br7 polyhedron is a distorted monocapped trigonal prism. Although the coordination number is smaller, the Br- radius is 1.99 Å as in the case of Pb(1)Br8. The cationic Pb(2)2+ radius is 1.10 Å. A smaller value in comparison with the tabular value (1.23 A for Pb2+(VII)8 is explained also by a covalent input to ionic bonds in bromide. Comparison of the obtained cationic radii and the Er3+ radius shows that the most probable Er position in mixed bromide is that of Pb(2). This can explain an increase of Er segregation coefficient as (x) increases. Table 1 shows that the compound changes symmetry of space group at x ) 0.35. At lower x values the KxRb1-xPb2Br5 structure is tetragonal (I4/mcm), whereas at larger x values it is monoclinic (P21/c). Data from Table 1 and Figures 1 and 2 show that
Figure 1. Dependence of the KxRb1-xPb2Br5 unit cell volume on composition. Points are experimental results and the solid curve is a result of simulation using the cubic equation V)1038 - 54x + 70x2 45x3.
Figure 2. Dependence of lattice parameters a, e, b, and c on composition for KxRb1-xPb2Br5 crystals. The axis order is explained in the text and in Figure 4. Dotted lines demonstrate changes in the axes order at structural transformation (tetragonal f monoclinic).
volume of the crystallographic unit cell and the b parameter do not change at structure transformation from the tetragonal to the monoclinic one at x ) 0.35. In Figure 3 the fragments of chains including two Pb polyhedrons are shown for both crystals. The fragment length is equal to b parameter. Here we see that an order of crystallographic axes changes at structure transformation (atetr f cmon and amon f ctetr). It is known that cation substitution for the isovalent ones, which are close in Mendeleev’s table, affects the size of the crystallographic cells, but does not change the relative position of the atoms. Given that atomic positions do not change considerably for K f Rb substitution, the cationic radii in mixed crystals can be determined. The analysis of cation radii changes (Figure 4) allows one to explain a reason for the structure change at x ) 0.35. This figure shows that tetragonal symmetry is advantageous for Rb atoms. They occupy a larger, symmetric cavity in the anionic lattice which corresponds better to Rb+ ionic radius. For Pb atoms the tetragonal structure is disadvantageous because the size of the Pb cavity is too large for Pb2+. The size of this cavity is noticeably larger than the average size of cavities for two Pb cations, Pb(1) and Pb(2), in the monoclinic crystal. In turn, the monoclinic structure is advantageous for Pb atoms but disadvantageous for Rb, which is to occupy a smaller K site.
2250
Crystal Growth & Design, Vol. 9, No. 5, 2009
Isaenko et al.
Figure 4. Dependence of the (K-Rb)+ and Pb(i)2+ cationic radii on composition for KxRb1-xPb2Br5.
Figure 3. A structural fragment which is similar both in tetragonal and monoclinic KxRb1-xPb2Br5 crystals. Above is a tetragonal cell, and below is a monoclinic cell. At left the axes of crystallographic cells for these crystals are shown.
The given data allow us to explain a sudden increase of the REI segregation coefficient in KxRb1-xPb2Br5 mixed crystals at tetragonal f monoclinic transformation. The Pb(2)2+ size in monoclinic structure is the closest to cation radii of REI. Thus, the Pb(2)2+ substitution for REI happens with a higher efficiency than Pb substitution in a relatively large cavity in a tetragonal structure.
4. Phase Transitions in Mixed Crystals Given that phase transition may be the reason for twins formation in monoclinic crystals, the dependence of the mmm - 2/m phase transition temperature on mixed crystal composition at x g 0.4 was studied. In order to reveal the phase transition, the polarization-optical observations were used and the angle of optical indicatrix rotation was measured using a polarization microscope Axiolab. In samples of variable composition in the x ) 0-0.3 range, single crystals of tetragonal symmetry were obtained. These are optically uniaxial crystals with a good extinction in cuts parallel to the optical axis. Such behavior is saved from 600 K to liquid nitrogen temperature. There are no phase transitions up to the melting temperature. Such compounds crystallize without defects. In the x ) 0.4-1 region, crystals of monoclinic symmetry were obtained. They are optically anisotropic with “direct” extinctions in (100) and (001) cuts, whereas in plates of the (010) cut a system of twins was visualized. Its components differ 2φ ≈ 3-4° in the extinction position. In pure KPb2Br5 (x ) 1), a twin structure with boundaries along [100] and [001] can
Figure 5. Temperature dependence of the turn angle of optical indicatrix φ(T) for KxRb1-xPb2Br5: crystals with x ) 1 (curve 1), x ) 0.5 (2), x ) 0.6 (3).
be seen. The width of the twins is 1 to 10 µm.9 The twin system becomes more regular as x decreases. At x e 0.3 single-domain large blocks, free of defects, are grown. Temperature dependence of the optical indicatrix rotation angle φ(T) around the [010] axis in an individual twin of monoclinic phase in KxRb1-xPb2Br5 crystals is given in Figure 5. Curve 1 shows the φ(T) dependence in KPB crystal. It is quite unusual. The φ angle is about 4° at room temperature. This value remains constant during heating and increases to 7° only near the phase transition. Afterward, its value decreases to zero at T0v ) 520 K. Further heating does not change the extinction position. At temperature above the phase transition point, crystal obtains orthorhombic symmetry 2/m - mmm. In KxRb1-xPb2Br5 samples with x ) 0.5 (curve 3) and x ) 0.4 (curve 2), an analogous temperature behavior of the indicatrix rotation angle versus can be observed, but the anomalies temperatures shift to higher values. Phase transition to orthorhombic phase for both compositions occurs approximately at T0 ≈ 620 K, about 100 K higher than in the case of pure KPb2Br5. Orthorhombic phase exists in a very narrow region of about 1-2 K wide, and further melting takes place.
Mixed KxRb1-xPb2Br5 Crystals
Figure 6. Phase diagram (T-x) of KxRb1-xPb2Br5.
The above experimental results allowed us to construct the phase (T-x) diagram for KxRb1-xPbBr5 solid solutions (Figure 6). There the region can be seen where substance with the I4/ mcm symmetry exists at 0 0.4) and a small area with the orthorhombic mmm symmetry. The line of P21/c (2/m) - mmm phase transitions approaches the boundary of the liquid aggregative state, but does not cross it. Thus the orthorhombic phase region narrows to 1-2 degrees in compounds with x ) 0.4-0.5. Given that there is a small distance between the melting point and the phase transition at x ) 0.5, crystallization from the supercooled melt in the temperature region of only monoclinic phase existence was arranged. Thus the phase transition and formation of the ferroelastic twins were avoided. As a result of such experiments, K0.5Rb0.5Pb2Br5 single crystals of high optical quality were obtained, with a high segregation REI coefficient k ≈ 0.15.
5. Conclusions The new single crystals of the KxRb1-xPb2Br5 set with 0 e x e 1, both pure and Er3+ doped, were obtained using the Bridgman technique. At 0 e x < 0.3 the compound was found
Crystal Growth & Design, Vol. 9, No. 5, 2009 2251
to crystallize in tetragonal symmetry, whereas at 0.3 e x e 1 symmetry was monoclinic. Detailed structural analysis revealed the reason for the structural transition and difference in REI segregation coefficients for tetragonal and monoclinic crystals. In monoclinic crystals, the first-order ferroelastic phase transition with the 2/m - mmm symmetry changes can be observed. This leads to formation of the twin structure in the monoclinic phase and to the deterioration of optical quality. In crystals of tetragonal symmetry, no phase transition takes place up to the melting temperature: Such crystals are defect-free. The dependence of phase transition temperature on composition in the set of KxRb1-xPb2Br5 crystals with 0 e x e 1 was revealed: This temperature grows as x decreases. At x ) 0.5 the phase transition temperature is close to the melting point, they differ only in several degrees. This allows crystals of optical quality to be grown in the temperature range where only monoclinic modification exists. The crystal undergoes no phase transition at cooling. The Er3+ segregation coefficient in mixed KxRb1-xPbBr5 crystals was found to increase as the potassium content increases. At 0.2 e x e 0.5 the REI concentration sufficient for the effective generation of stimulated emission is reached. Supporting Information Available: This information is available free of charge via the Internet at http://pubs.acs.org.
References (1) Rademaker, K.; Heumann, E.; Huber, G.; Payne, S. A.; Krupke, W. F.; Isaenko, L. I.; Burger, A. Opt. Lett. 2005, 30 (7), 729–731. (2) Mel’nikova, S. V.; Isaenko, L. I.; Pashkov, V. M.; Pevnev, I. V. Phys. Solid State 2006, 48, 2152–2156. (3) Isaenko, L. I.; Merkulov, A. A.; Tarasova, A. Yu.; Pashkov, V. M.; Drebushchak, V. A. J. Therm. Anal. Calorim., DOI 9089, in press. (4) Powell, H. M.; Tasker, H. S. J. Chem. Soc. 1937, 119–123. (5) Cola, M.; Massariti, V.; Richard, R.; Siristri, C. Z. Naturforsch. 1971, A26, 1328–1332. (6) Merkulov, A; Isaenko, L. I.; Pashkov, V. M.; Mazur, V. G.; Virovets, A. V.; Naumov, D. Yu. Zh. Strukt. Khim. 2005, 6 (1), 106–110. (7) Powell, H. M.; Tasker, H. S. J. Chem. Soc. 1937, 119–123. (8) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767. (9) Mel’nikova, S. V.; Isaenko, L. I.; Pashkov, V. M.; Pevnev, I. V. Phys. Solid State 2005, 47, 319–325. .
CG8010162
