AbstracL An x-ray singleclystal determination of ihe CuO stmture has been made at 196 K, i.e. below the N6el temperature 230 K, and, as a check, the crystal smclure a1 m m temperature was also determined. The correct space p u p for lhe structure at bolh temperatures was found lo be C c . Earlier mulls of magnnk and neutmn diiimction measuremenls can be explained as antifemmagnetic coupling betwgn copper atoms via oxygen (superexchange) in chains running in the [I 0-11 direction. The s " a l results show changes with temperature in Cu-0 distances in these chains in each -Cu-(M1group lhe longer distance is increased and the shorter decreased when passing from 196 K to m m temperature. This implies a weaW antiferromagnetic mupiing at room temperature. The re6nement of CuO-stmctun at r w m temperature reponed earlier by Asbrink and Norrby (1970) showed the symmetry to be CZ/e. An attempt to refine CUO in the space group C e with the old data was not successful. The diffeerent m u l k obtained Hith different aystals are tentatively explained from published observations regarding valence fluctuations in CuO and non-stoichiometry caused by cation vacancies.

Synchrotron-radiation X-ray diffraction studies of CaF2 at high pressures have been performed on a powder sample up to 45 GPa and on a single-crystal sample up to 9.4 GPa. The bulk modulus of the low-pressure phase was determined to be B o = 87 (5) GPa. A phase transition was observed at about 9.5 GPa. The transition is accompanied by a volume contraction of 11%. The high-pressure phase is orthorhombic PbC12 type (space group Pbnm). The sample only partially reverts to the low-pressure phase upon release of pressure.

X-ray photoelectron spectroscopy, magnetic measurements, and a single-crystal x-ray structure determination at normal pressure have shown that Jahn-Teller active manganese ions in ZnMn 2 O 4 are present in one valence state ͑III͒ on the octahedral sites of the spinel structure. The high-pressure behavior of ZnMn 2 O 4 was investigated up to 52 GPa using the energy-dispersive x-ray diffraction technique and synchrotron radiation. The structural first-order phase transition from the body-centered to primitive-tetragonal cell takes place at P c ϭ23 GPa. The high-pressure phase is metastable down to normal pressure. The c/a ratio reduces from 1.62 to 1.10 above P c and remains nearly pressure independent in the high-pressure phase. The transition is attributed to the changes in electron configuration of the Mn 3ϩ ions. According to the crystal field theory, the e g electron of octahedrally coordinated Mn 3ϩ is either in the d z 2 orbital or in the d x 2 Ϫy 2. In the first configuration the MnO 6 octahedron will be elongated and this is the case at normal pressure, while the second configuration gives the flattened octahedron. In the high-pressure phase some proportion of the e g electrons of the Mn 3ϩ ions is moved to the d x 2 Ϫy 2 level, which is revealed as an abrupt fall of observed magnitude of the distortion of the bulk crystal above P c . ͓S0163-1829͑99͒08341-1͔PHYSICAL REVIEW B

Single crystal x-ray diffraction, x-ray photoelectron spectroscopy and magnetic susceptibility measurements at normal pressure have shown that, in spite of two Jahn-Teller active ions in CuMn2O4, the crystal is cubic with partly inverse spinel structure, the inversion parameter being \mbox{$x = 0.8$}. The cation configuration at normal pressure was determined as Cu0.2+Mn2+0.8[Cu2+0.8Mn3+0.2Mn4+1.0]O4. The high-pressure behaviour of the crystal was investigated up to 30 GPa using the energy dispersive x-ray diffraction technique and synchrotron radiation. A first-order phase transition connected with a tetragonal distortion takes place at Pc = 12.5 GPa, the c/a ratio being 0.94 at P = 30 GPa. The high-pressure phase has been described in terms of ligand field theory and explained by the changes to the valence and electronic configuration of the metal ions, leading to the formula Cu2+0.2Mn3+0.8[Cu2+0.8Mn3+1.2]O4. The electron configuration of the tetrahedrally coordinated Cu2+ and Mn3+ is (e4)t5 and e2t2, respectively. On the other hand, the electron configuration of Cu2+ located at octahedral sites is (t62g)e3g. While six electrons with antiparallely aligned spins occupy the triplet (t62g), three electrons on the orbital eg can be distributed in two ways (double degeneracy): (dx2-y2)1(dz2)2 and (dx2-y2)2(dz2)1. The first alternative leads to an axially elongated octahedron; the second one causes flattening of the octahedron. The contraction of the c axis indicates, that in the high-pressure phase the second configuration with unpaired electron on the dz2 orbital occurs. A similar effect of the octahedral contraction brings the orbital degeneracy of Mn3+ with the t32ge1g distribution. It follows that at high pressure the ligand field forces the two metals to take the valences that they show in the parent oxides CuO and Mn2O3.

In situ x-ray diffraction and Raman spectroscopy have been carried out to pressures of 93.6 and 63.2 GPa, respectively, to explore the pressure-induced phase transformation of CoFe 2 O 4 spinel. CoFe 2 O 4 adopts a distorted tetragonal spinel structure at one atmosphere. At a pressure of ϳ32.5 GPa, both x-ray diffraction and Raman spectroscopy indicate that CoFe 2 O 4 transforms to the orthorhombic CaFe 2 O 4 structure, which remains stable to at least 93.6 GPa. The bulk modulus (K 0 ) of the tetragonal and the high-pressure polymorphs were calculated to be 94͑12͒ and 145͑16͒ GPa, respectively, with KЈϵ4. Upon release of pressure the orthorhombic phase persists and appears to be structurally metastable. At zero pressure, laser induced heating leads to a significant transformation back to the tetragonal phase. The high-pressure orthorhombic phase at one atmosphere is 14.7% denser than the tetragonal phase.