INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 17 (2005) 6285–6300 doi:10.1088/0953-8984/17/39/013 High pressure effects on the structural and vibrational properties of antiferromagnetic KFe(MoO 4 ) 2 M M˛ aczka 1 , A Pietraszko 1 , G D Saraiva 2 , A G Souza Filho 2 , W Paraguassu 2 , V Lemos 2 , C A Perottoni 3,4 , M R Gallas 4 , P T C Freire 2 , P E Tomaszewski 1 , F E A Melo 2 , J Mendes Filho 2 and J Hanuza 1,5 1 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, PO Box 1410, 50-950 Wroclaw 2, Poland 2 Universidade Federal do Cear´ a, Departamento de F´ ısica, PO Box 6030, CEP 60455-900, Fortaleza, CE, Brazil 3 Universidade de Caxias do Sul, Centro de Ciˆ encias Exatas e Tecnologia, Departamento de F´ ısica e Qu´ ımica, 95070-560 Caxias do Sul, RS, Brazil 4 Universidade Federal do Rio Grande do Sul, Instituto de F´ ısica, Laborat´ orio de Altas Press˜ oes e Materiais Avan¸ cados, 91501-970 Porto Alegre, RS, Brazil 5 Department of Bioorganic Chemistry, Faculty of Industry and Economics, Wroclaw University of Economics, 118/120 Komandorska Street, 53-345 Wroclaw, Poland Received 21 June 2005, in final form 9 August 2005 Published 16 September 2005 Online at stacks.iop.org/JPhysCM/17/6285 Abstract The crystal structure of the paraelastic phase of KFe(MoO 4 ) 2 at 360 K was reinvestigated and high pressure Raman scattering experiments were performed on this material. The studies indicated that this molybdate crystallizes in the P ¯ 3m1 structure above 312 K. At room temperature the structure is monoclinic and it transforms under pressure into P ¯ 3m1, P ¯ 3c1 and low symmetry phases at 0.25, 1.3 and 1.6 GPa, respectively. The phase transitions observed at 0.25 and 1.6 GPa are irreversible whereas the 1.3 GPa transition is reversible. The lattice dynamics calculations performed for the P ¯ 3m1 phase allowed us to obtain an assignment of observed modes and helped us to obtain insight into the mechanism driving the structural changes occurring in this material. The x-ray study of the highest pressure phase, recovered during the decompression experiment, shows that the crystal structure of this phase is monoclinic or triclinic. When this phase is subjected to heat treatment at 673 K, it either transforms into another phase or decomposes. 1. Introduction KFe(MoO 4 ) 2 belongs to the family of layered trigonal molybdates and tungstates with chemical formula M + M 3+ (MoO 4 ) 2 , where M + = Na, K, Rb, Cs and M 3+ = Al, Sc, Cr, Fe [1–4]. These materials have been the subject of many studies owing to the richness of structural instabilities induced either by heat or hydrostatic pressure [1–6]. Recently, KFe(MoO 4 ) 2 and 0953-8984/05/396285+16$30.00 © 2005 IOP Publishing Ltd Printed in the UK 6285