Electronic origin of negative thermal expansion in VOPO.

Negative volume expansion between 620 and 800 K in VOPO is discovered to be of electronic origin due to the charge ordering transition at 605 K. Domain reorientation and coexistence of the low and high temperature phases close to the transition are observed in X-ray diffraction data from single crystals grown by chemical vapour transport.

Co-emergence of magnetic order and structural fluctuations in magnetite.

The nature of the Verwey transition occurring at T ≈ 125 K in magnetite (FeO) has been an outstanding problem over many decades. A complex low temperature electronic order was recently discovered and associated structural fluctuations persisting above T are widely reported, but the origin of the underlying correlations and hence of the Verwey transition remains unclear. Here we show that local structural fluctuations in magnetite emerge below the Curie transition at T ≈ 850 K, through X-ray pair distribution function analysis.

Charge Order and Negative Thermal Expansion in VOPO.

The semivalent oxyphosphate VOPO is found to have long-range V/V charge ordering up to 605 K where a monoclinic to tetragonal structural transition and a switch from positive to negative thermal expansion are observed. V-V bonding within orbital polymer chains is proposed as the key factor in the novel switch of thermal expansion behavior, as loss of V-V bonding enables transverse oxygen motions to dominate the thermal expansion at high temperatures. Ferrimagnetic order of V spin up and V spin down states is observed below a magnetic ordering transition at 164 K, and susceptibility measurements evidence local spin pairing correlations to higher temperatures.

The spin glass delafossite CuFe(0.5)V(0.5)O(2): a dipolar glass?

The ferroelectric and spin glass properties of CuFe(0.5)V(0.5)O(2) have been studied.