CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF CUMOO4 AT LOW-TEMPERATURE (GAMMA-PHASE)

The crystal structure and magnetic properties of a new modification of copper molybdate (gamma-CuMoO4) are reported, which is stable below 2 00 K. Single-crystal X-ray diffraction revealed triclinic symmetry (P (1) over bar) with a = 9.699(9) Angstrom, b = 6.299(6) Angstrom, c = 7 .966(7) Angstrom, alpha = 94.62(4)degrees, beta = 103.36(4)degrees, ga mma = 103.17(4)degrees at 180 K and Z = 6. All Cu- and Mo-ions are sur rounded by distorted oxygen octahedra. In contrast to the high-pressur e modification CuMoO4-II, which is of a triclinic distorted wolframite structure, the octahedra are not closed packed. This is also in contr ast to the triclinic (P (1) over bar) room-temperature phase (alpha) a t standard pressure with tetrahedrally coordinated Mo-ions and with a = 9.901(3) Angstrom b = 6.786(2) Angstrom, c = 8.369(3) Angstrom, alph a = 101.13(1)degrees, beta = 96.88(1)degrees and gamma = 107.01(1)degr ees. The new crystal structure of gamma-CuMoO4 is extensively discusse d and compared with those of AlNbO4 and alpha-CoMoO4. The transition b etween alpha- and gamma-phase is of first order and a wide temperature range of coexistence is observed in powder experiments. The relative amount of both phases depends on both temperature and cooling rate. No long range magnetic ordering was observed down to 1.8 K by SQUID meas urements. The magnetic properties of gamma-CuMoO4 can be explained by a chain-like arrangement of copper-moments deduced from the crystal st ructure. If only interactions within such chains are considered, the c alculations based on an Ising-model are in good agreement with experim ental data. Copyright (C) 1996 Elsevier Science Ltd