Olivier Crottaz

Single crystal growth, structure refinement, ferroelastic domains and phase transitions of the hausmannite CuCr2O4

Abstract With a view to studying the optical, structural and magnetoelectric properties, single crystals of the hausmannite CuCr2O4 have been grown by a high temperature solution method, based on the thermal decomposition of K2Cr2O7 in the presence of CuO. X-ray powder refinements at room temperature indicate that the crystal structure of CuCr2O7 can be accurately described by the centrosymmetric space group I41/amd. Crystal cuts have been examined by means of polarized light microscopy in reflection, revealing ferroelastic domain structures consistent with the tetragonal symmetry. The phase transitions in the copper chromite have been studied both by in situ optical domain observations and by DTA and DSC measurements. The ferroelastic phase transition from the tetragonal to the cubic phase has been found to take place at 853 K and to be of first order. The magnetic phase transition has been revealed by a DSC peak at 157 K with a small enthalpy of 0.2 J/g.

Low temperature specific heat capacity of 3d transition metal chlorine boracites (T3B7O13Cl; T=Cr, Mn, Fe, Co, Ni, Cu, Zn or Mg)

The heat capacities of eight chlorine boracites T3B7O13Cl (T=Cr, Mn, Fe, Co, Ni, Cu, Zn or Mg) have been measured in the temperature range 2 to 100 K. Magnetic phase transitions occur below 20 K in the compounds studied except in the two non-magnetic substances Zn3B7O13Cl and Mg3B7O13Cl. The magnetic specific heat capacities give information on magnetic ground state of the transition metals and the entropy related to the phase transitions.

Magnetic ordering of Mn3B7O13X (X = Cl, Br, I) boracites determined by magnetic measurements and neutron diffraction

Abstract A new method for the preparation of Mn3 11B7O13I powder sample, used for neutron diffraction, is presented. Magnetization measurements on powder samples of Mn3B7O13X (X=C1, Br, I) have shown that their Neel temperatures are 11±0.5 K for Mn3B7O13Cl, 14±0.5 K for Mn3B7O13Br and 26±0.5 K for Mn3B7O13I. The crystal and the magnetic structures of Mn3B7O13I have been investigated by neutron diffraction. The neutron diffraction pattern in the paramagnetic phase at 35 K was refined in space group Pca211′. The neutron diffraction data confirm the onset of magnetic ordering at 26 K. The magnetic structure has been determined at 1.5 K in the magnetic space group Pc′a21′. The magnetic moments of the three manganese sites order with a two-dimensional canted spin arrangement. The moments of two sites point along c with μz(Mn2) = μz(Mn3) = 3.8(2) μ B /atom while the Mn(l) site has a magnetic moment of 5.4(2) μ B /atom located in the (101) orthorhombic plane, forming an angle of 12.0(3) degrees with the a direct...

Jumping crystals of the spinels NiCr 2 O 4 and CuCr 2 O 4

The crystal structures of cubic and tetragonal NiCr 2 O 4 and of tetragonal CuCr 2 O 4 have been refined and their cell parameters have been measured, using single-crystal and powder X-ray diffraction. It has been observed that the differences in the Jahn–Teller distortions in both compounds are reflected in their M 2+ –O tetrahedral environments and in their cell parameters. A calculation of the atomic shifts during the tetragonal–cubic phase transition showed that the average shifts for the oxygen atoms are 0.10(1) and 0.04(1) A for the nickel and chromium atoms, respectively. Crystals of both compounds jump when they go through the phase transition. This behaviour is especially spectacular for NiCr 2 O 4 since the phase transition takes place at 320 K, this temperature being reached when the crystals are illuminated. A comparison with organic jumping crystals is presented, and characteristics of the chromite crystals are discussed.

On various types of piezoelectric anisotropy in polydomain boracite crystals

Abstract The paper is devoted to a determination of effective piezoelectric coefficients d3j poly and e3j poly (j = 1, 2, 3) of boracite crystals with 180° domain structure. Some features of these coefficients and their anisotropy are studied for different domain volume concentrations and/or temperatures.