Synthesis and characterization of mixed-metal germanates

Mixed-metal germanates with the general formula ABB’Ge4O12 have been synthesised using the ceramic method and their properties characterised by X-ray diffraction, neutron diffraction, dc and ac magnetometry, specific heat and Mossbauer spectroscopy. DFT calculations were conducted on two compounds ZrM2Ge4O12 (M = Mn, Co). All compositions adopt a tetragonal structure (space group P4/nbm with the unit-cell parameters a = b ~ 2c). The cation A occupies the 2b site and is coordinated by eight oxide ions at the corners of a square antiprism whereas B and B' are disordered over the 4f site which is at the centre of a distorted octahedron of oxide ions. They can be considered to lie in layers perpendicular to the [001] axis. These layers are separated from each other by layers containing [Ge4O12]8- rings, each ring being comprised of four vertex-sharing GeO4 tetrahedra.
CeMn2−xCoxGe4O12 (x = 0.0, 0.5, 1.0, 1.5, and 2.0) become anti-ferromagnetic (x = 0.0, 1.5, 2.0) or weakly ferromagnetic (x = 0.5, 1.0) at 4.2 ≤ T ≤ 7.6 K. The ordered moments lie along [001] when x = 0.0 and in the (001) plane otherwise. The unit cell doubles along [001] when x = 1.5 and 2.0, but the doubling is lost when a first-order metamagnetic transition occurs on the application of a 10 kOe magnetic field. The ordered moments at 1.6 K for x = 0.0 and 2.0 are 4.61(2) and 2.58(2) μB, respectively; the corresponding effective moments in the paramagnetic phase are 5.91 and 5.36 μB. CeM1.5M'0.5Ge4O12 (M = Mn, Co; M' = Zn, Ni, Cu) show similar magnetic properties as CeM2Ge4O12 albeit with changes in the Néel temperature and Weiss constant due to the differences between the cations used.
ZrMn2−xCoxGe4O12 (x = 0.0, 0.5, 1.0, 1.5, and 2.0) show long-range magnetic order with transition temperatures, TC, in the range 2 ≤ TC/K ≤ 10. The underlying magnetic structure is the same in each case but the ordered spins lie along [001] when x = 0.0 and in the (001) plane for all other compositions. In all cases the magnetically-ordered phase is a weak ferromagnet although the magnitude of the spontaneous magnetisation and the strength of the coercive field are composition-dependent. The magnetic structure can be rationalized by considering the strengths of the interactions along the distinct M–O–Ge–O–M superexchange pathways in the crystal and the observed magnetic structure is entirely consistent with the predictions of ab initio calculations.
LnFeMGe4O12 (Ln = Y, Eu, Gd, Lu; M = Mn, Zn) show long-range antiferromagnetic order with transition temperatures 15 ≤ TN/K &;e; 30. The magnetic structure is the same in each case and consists of an A-type ordering of (001) planes; the ordered spins lie in the (001) plane. Comparison with isostructural compounds leads to the conclusion that subtle structural changes play a greater role than the electronic configuration of the cation in determining the magnetic structure.
Ln2MGe4O12 (Ln = Gd-Yb; M = Ca, Mn, Co) and LnBCoGe4O12 (B = Sc or Lu) show various magnetic behaviours. The calcium-, holmium- and erbium-containing compositions remain paramagnetic down to 2 K; the other cases show a transition at the temperatures ~ 4 K. Dy2CoGe4O12 and DyScCoGe4O12 behave as spin glasses and the terbium- and gadolinium-cobalt-containing compounds show long-range magnetic order. Tb2MnGe4O12 shows a weakly ferromagnetic phase and Gd2MnGe4O12, Dy2MnGe4O12 are antiferromagnets. The data can be rationalized qualitatively in terms of the interplay between magnetic anisotropy and crystal field effects.

Advisor: Battle, P