Realizamos experimentos de espectroscopia Raman em condic¸˜oes de altas press˜oes para o molibdato de ´ıtrio. Como suporte para interpretar os experimentos, executamos c´alculos da dinˆamica de rede utilizando o modelo do ´ıon r´ıgido com um potencial harmˆonico adicionado como fator de correc¸˜ao. A partir dos resultados desses c´alculos e da aplicac¸˜ao da Teoria de Grupos, relacionamos os movimentos atˆomicos com os modos observados no espectro Raman do material em condic¸˜ao ambiente de temperatura e press˜ao.
Em nosso experimento realizado para o Y2(MoO4)3 (fase anidra), observamos um novo
modo (978 cm−1) em torno de 0,3 GPa, interpretando isto `a ocorrˆencia de uma transic¸˜ao de fase estrutural. Desde que a literatura relata para o material isoestrutural Sc2(MoO4)3 uma
transic¸˜ao de fase ortorrˆombica para monocl´ınica em 0,29 GPa, sugerimos uma transic¸˜ao de fase similar para o Y2(MoO4)3. Para press˜oes a partir de 1,4 GPa, notamos que os modos de
flex˜ao colapsam numa ´unica banda e, de forma geral, todos os modos perdem resoluc¸˜ao com o aumento da press˜ao, tornando-se mais largos e menos intensos. Associamos essa mudanc¸a de comportamento a uma transic¸˜ao para uma fase desordenada de alta press˜ao. Destacamos que um dos modos de flex˜ao de coeficiente α negativo deixa de ser distingu´ıvel no espectro desse ponto em diante e a banda resultante do colapso dos modos de flex˜ao possui coeficiente α positivo. Isto pode indicar o limite de press˜ao para o qual o Y2(MoO4)3 exibe o fenˆomeno
de expans˜ao t´ermica negativa, caso a perda da fase ortorrˆombica n˜ao determine esse limite por si mesma. A partir de 2,4 GPa, os espectros apresentam apenas trˆes bandas degeneradas, que ganham largura e perdem intensidade ao longo do regime de compress˜ao. Interpretamos essa observac¸˜ao qualitativa como a perda da correlac¸˜ao de longo alcance e desordenamento estrutural crescente do material com a press˜ao. ´E tamb´em poss´ıvel que press˜oes acima do limite estudado nesse trabalho possam induzir completa amorfizac¸˜ao no Y2(MoO4)3. Recuperada a press˜ao
ambiente, o material retornou `a fase alcanc¸ada em 2,4 GPa durante a compress˜ao, nos levando a concluir que a transic¸˜ao para a fase desordenada ´e irrevers´ıvel.
No experimento realizado para o Y2(MoO4)3·xH2O (x< 3, fase hidratada), observamos
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dos obtidos para as fases hidratada e anidra, percebemos que o desordenamento estrutural com o aumento da press˜ao hidrost´atica ´e mais acentudado na fase hidratada. Ent˜ao, ´e vis´ıvel que a limitac¸˜ao das vibrac¸˜oes dos tetraedros MoO4 provocada pelas mol´eculas de ´agua acelera o
processo de desordenamento induzido pela press˜ao. Al´em disso, as mol´eculas de ´agua suprim- iram a transic¸˜ao estrutural que observamos no Y2(MoO4)3. A descompress˜ao mostrou que a
transic¸˜ao “ordem-desordem” na fase hidratada tamb´em ´e irrevers´ıvel.
A literatura associa a fase ortorrˆombica nos materiais Y2(MoO4)3 ao fenˆomeno de ex-
pans˜ao t´ermica negativa. Logo, determinar os limites de temperatura e press˜ao para os quais o Y2(MoO4)3 mostra-se ortorrˆombico pode ser crucial para sua utilizac¸˜ao em aplicac¸˜oes tec-
nol´ogicas, nos levando a sugerir a realizac¸˜ao de experimentos de espectroscopia Raman em condic¸˜oes de baixas temperaturas, que tanto podem determinar a ocorrˆencia de transic¸˜oes de fase estruturais como revelar outros aspectos dos fˆonons do Y2(MoO4)3. Sugerimos tamb´em a
realizac¸˜ao de experimentos de difrac¸˜ao de raios-X em condic¸˜oes de altas press˜oes, com o obje- tivo de determinar com exatid˜ao a fase estrutural do Y2(MoO4)3que observamos para press˜oes
em torno de 0,3 GPa, bem como avaliar a perda da correlac¸˜ao de longo alcance desse material com o aumento da press˜ao e confirmar a ocorrˆencia de amorfizac¸˜ao do mesmo para press˜oes superiores a 10 GPa.
Bibliografia
1 D. A. Long, “The Raman Effect,” John Wiley Sons LTD, 2002. 2 C. Kittel, “Introduction to Solid State Physics,” LTC, 2009.
3 C. Guzman-Afonso, C. Gonzalez-Silgo, J. Gonzalez-Platas, M. Eulalio Torres, A. Diego Lozano-Gorrin, N. Sabalisck, V. Sanchez-Fajardo, J. Campo, and J. Rodriguez-Carvajal, “Structural investigation of the negative thermal expansion in yttrium and rare earth molybdates,” JOURNAL OF PHYSICS-CONDENSED MATTER, vol. 23, AUG 17 2011. 4 S. Sumithra and A. M. Umarji, “Negative thermal expansion in rare earth molybdates,” SOLID STATE SCIENCES, vol. 8, pp. 1453–1458, DEC 2006.
5 J. Evans, T. Mary, and A. Sleight, “Negative thermal expansion materials,” PHYSICA B, vol. 241, pp. 311–316, DEC 1997.
6 T. Varga, A. Wilkinson, C. Lind, W. Bassett, and C. Zha, “High pressure synchrotron x-ray powder diffraction study Of Sc2Mo3O12 and Al2W3O12,” JOURNAL OF PHYSICS- CONDENSED MATTER, vol. 17, pp. 4271–4283, JUL 13 2005.
7 B. Marinkovic, P. Jardim, R. de Avillez, and F. Rizzo, “Negative thermal expansion in Y2Mo3O12,” SOLID STATE SCIENCES, vol. 7, pp. 1377–1383, NOV 2005.
8 P. Forster and A. Sleight, “Negative thermal expansion in Y2W3O12,” INTERNATIONAL JOURNAL OF INORGANIC MATERIALS, vol. 1, pp. 123–127, AUG 1999.
9 J. Evans and T. Mary, “Structural phase transitions and negative thermal expansion in Sc-2(MoO4)(3),” INTERNATIONAL JOURNAL OF INORGANIC MATERIALS, vol. 2, pp. 143–151, FEB 2000.
10 J. Evans, T. Mary, and A. Sleight, “Negative thermal expansion in Sc-2(WO4)(3),” JOURNAL OF SOLID STATE CHEMISTRY, vol. 137, pp. 148–160, APR 1998.
11 S. Sumithra, A. Tyagi, and A. Umarji, “Negative thermal expansion in Er2W3O12 and Yb2W3O12 by high temperature X-ray diffraction,” MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, vol. 116, pp. 14–18, JAN 15 2005.
12 J. Evans, Z. Hu, J. Jorgensen, D. Argyriou, S. Short, and A. Sleight, “Compressibility, phase transitions, and oxygen migration in zirconium tungstate, ZrW2O8,” SCIENCE, vol. 275, pp. 61–65, JAN 3 1997.
13 J. Jorgensen, Z. Hu, S. Teslic, D. Argyriou, S. Short, J. Evans, and A. Sleight,
“Pressure-induced cubic-to-orthorhombic phase transition in ZrW2O8,” PHYSICAL REVIEW B, vol. 59, pp. 215–225, JAN 1 1999.
40
14 T. Ravindran, A. Arora, and T. Mary, “Anharmonicity and negative thermal expansion in zirconium tungstate,” PHYSICAL REVIEW B, vol. 67, FEB 1 2003.
15 B. A. Marinkovic, M. Ari, R. R. de Avillez, F. Rizzo, F. F. Ferreira, K. J. Miller, M. B. Johnson, and M. A. White, “Correlation between AO(6) Polyhedral Distortion and Negative Thermal Expansion in Orthorhombic Y(2)Mo(3)O(12) and Related Materials,” CHEMISTRY OF MATERIALS, vol. 21, pp. 2886–2894, JUL 14 2009.
16 S. Sumithra, U. V. Waghmare, and A. M. Umarji, “Anomalous dynamical charges, phonons, and the origin of negative thermal expansion in Y2W3O12,” PHYSICAL REVIEW B, vol. 76, JUL 2007.
17 S. Sikka, “Negative thermal expansion and its relation to high pressures,” JOURNAL OF PHYSICS-CONDENSED MATTER, vol. 16, pp. S1033–S1039, APR 14 2004.
18 L. BRIXNER, “PI-GMO - ANOTHER MODIFICATION OF GD2(MO04)3,” MATERIALS RESEARCH BULLETIN, vol. 7, no. 9, pp. 879–&, 1972.
19 L. BRIXNER, A. SLEIGHT, and M. LICIS, “CELL DIMENSIONS OF MOLYBDATES LA2(MOO4)3, CE2(MOO4)3, PR2(MOO4)3, AND ND2(MOO4)3,” JOURNAL OF SOLID STATE CHEMISTRY, vol. 5, no. 2, pp. 247–&, 1972.
20 A. JAYARAMAN, S. SHARMA, and S. WANG, “PRESSURE-INDUCED AMOR- PHIZATION OF GD2(MOO4)3 - A HIGH-PRESSURE RAMAN INVESTIGATION,” PRAMANA-JOURNAL OF PHYSICS, vol. 40, pp. 357–365, MAY 1993.
21 W. Paraguassu, M. Maczka, A. Souza, P. Freire, J. Mendes, F. Melo, L. Macalik, L. Gerward, J. Olsen, A. Waskowska, and J. Hanuza, “Pressure-induced structural transformations in the molybdate Sc-2(MoO4)(3),” PHYSICAL REVIEW B, vol. 69, MAR 2004.
22 M. Maczka, W. Paraguassu, A. Souza, P. Freire, J. Mendes, F. Melo, and J. Hanuza, “High-pressure Raman study of Al-2(WO4)(3),” JOURNAL OF SOLID STATE CHEMISTRY, vol. 177, pp. 2002–2006, JUN 2004.
23 S. Karmakar, S. Deb, A. Tyagi, and S. Sharma, “Pressure-induced amorphization in Y-2(WO4)(3): in situ X-ray diffraction and Raman studies,” JOURNAL OF SOLID STATE CHEMISTRY, vol. 177, pp. 4087–4092, NOV 2004.
24 H. Liu, R. Secco, N. Imanaka, and G. Adachi, “X-ray diffraction study of pressure-induced amorphization in Lu-2(WO4)(3),” SOLID STATE COMMUNICATIONS, vol. 121, no. 4, pp. 177–180, 2002.
25 V. Dmitriev, V. Sinitsyn, R. Dilanian, D. Machon, A. Kuznetsov, E. Ponyatovsky, G. Lucazeau, and H. Weber, “In situ pressure-induced solid-state amorphization in Sm- 2(MoO4)(3), Eu-2(MoO4)(3) and Gd-2(MoO4)(3) crystals: chemical decomposition scenario,” JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, vol. 64, pp. 307–312, FEB 2003. 26 I. Shmyt’ko, E. Kudrenko, V. Sinitsyn, B. Red’kin, and E. Ponyatovsky, “Features of the pressure-induced phase transitions in Eu-2(MoO4)(3) single crystals,” JETP LETTERS, vol. 82, no. 7, pp. 409–412, 2005.
RAMAN AND X-RAY-DIFFRACTION STUDY,” JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, vol. 54, pp. 827–833, JUL 1993.
28 A. Jayaraman, S. Sharma, Z. Wang, and S. Wang, “Pressure-induced amorphization in the alpha-phase of Nd-2(MoO4)(3) and Tb-2-(MoO4)(3),” SOLID STATE COMMUNICATIONS, vol. 101, pp. 237–241, JAN 1997.
29 R. Secco, H. Liu, N. Imanaka, and G. Adachi, “Pressure-induced amorphization in negative thermal expansion Sc-2(WO4)(3),” JOURNAL OF MATERIALS SCIENCE LETTERS, vol. 20, no. 14, pp. 1339–1340, 2001.
30 S. Sharma and S. Sikka, “Pressure induced amorphization of materials,” PROGRESS IN MATERIALS SCIENCE, vol. 40, no. 1, pp. 1–77, 1996.
31 C. Perottoni and J. da Jornada, “Pressure-induced amorphization and negative thermal expansion in ZrW2O8,” SCIENCE, vol. 280, pp. 886–889, MAY 8 1998.
32 E. J. Liang, H. L. Huo, Z. Wang, M. J. Chao, and J. P. Wang, “Rapid synthesis of A(2)(MoO(4))(3) (A = Y(3+) and La(3+))with a CO(2) laser,” SOLID STATE SCIENCES, vol. 11, pp. 139–143, JAN 2009.
33 S. D. Gates and C. Lind, “Polymorphism in yttrium molybdate Y2Mo3O12,” JOURNAL OF SOLID STATE CHEMISTRY, vol. 180, pp. 3510–3514, DEC 2007.
34 J. Peng, M. M. Wu, F. L. Guo, S. B. Han, Y. T. Liu, D. F. Chen, X. H. Zhao, and Z. Hu, “Crystal structure and negative thermal expansion of solid solution Y(2)W(3-x) Mo (x) O(12),” JOURNAL OF MATERIALS SCIENCE, vol. 46, pp. 5160–5164, AUG 2011.
35 P. Forster, A. Yokochi, and A. Sleight, “Enhanced negative thermal expansion in Lu2W3O12,” JOURNAL OF SOLID STATE CHEMISTRY, vol. 140, pp. 157–158, OCT 1998. 36 E. Liang, H. Huo, J. Wang, and M. Chao, “Effect of water species on the phonon modes in orthorhombic Y-2(MoO4)(3) revealed by Raman spectroscopy,” JOURNAL OF PHYSICAL CHEMISTRY C, vol. 112, pp. 6577–6581, APR 24 2008.
37 E. Le Ru and P. Etchegoin, “Pinciples of Surface Enhanced Raman Spectroscopy and related plasmnic effects,” Elsevier, 2009.
38 M. Maczka, A. G. Souza Filho, W. Paraguassu, P. T. C. Freire, J. Mendes Filho, and J. Hanuza, “Pressure-induced structural phase transitions and amorphization in molybdates and tungstates,” To be published, 2011.
39 M. Maczka, E. Kokanyan, and J. Hanuza, “Vibrational study and lattice dynamics of disordered NaBi(WO4)(2),” JOURNAL OF RAMAN SPECTROSCOPY, vol. 36, pp. 33–38, JAN 2005.
40 M. Maczka, A. Pietraszko, G. Saraiva, A. Souza, W. Paraguassu, V. Lemos, C. Perottoni, M. Gallas, P. Freire, P. Tomaszewski, F. Melo, J. Mendes, and J. Hanuza, “High pressure effects on the structural and vibrational properties of antiferromagnetic KFe(MoO4)(2),” JOURNAL OF PHYSICS-CONDENSED MATTER, vol. 17, pp. 6285–6300, OCT 5 2005.
42
41 A. JAYARAMAN, “DIAMOND ANVIL CELL AND HIGH-PRESSURE PHYSICAL INVESTIGATIONS,” REVIEWS OF MODERN PHYSICS, vol. 55, no. 1, pp. 65–108, 1983. 42 H. MAO, J. XU, and P. BELL, “CALIBRATION OF THE RUBY PRESSURE GAUGE TO 800-KBAR UNDER QUASI-HYDROSTATIC CONDITIONS,” JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH AND PLANETS, vol. 91, pp. 4673–4676, APR 10 1986.
43 J. Gale, “GULP: A computer program for the symmetry-adapted simulation of solids,” JOURNAL OF THE CHEMICAL SOCIETY-FARADAY TRANSACTIONS, vol. 93, pp. 629–637, FEB 21 1997.
44 E. Dowty, “Vibrational interactions of tetrahedra in silicate glasses and crystals,” Physics and Chemistry of Minerals, vol. 14, pp. 542–552, 1987.