F. Büllesfeld

Synchrotron-radiation study of the two-leg spin-ladder (VO)2P2O7 at 120 K

The crystal structure of the ambient-pressure phase of vanadyl pyrophosphate, (VO)2P2O7, has been precisely determined at 120 K from synchrotron X-ray diffraction data measured on a high-quality single crystal. The structure refinement unambiguously establishes the orthorhombic space group Pca2(1) as the true crystallographic symmetry. Moreover, it improves the accuracy of previously published atomic coordinates by one order of magnitude, and provides reliable anisotropic displacement parameters for all atoms. Along the a axis, the structure consists of infinite two-leg ladders of vanadyl cations, (VO)2+, which are separated by pyrophosphate anions, (P2O7)4-. Parallel to the c axis, the unit cell comprises two alternating crystallographically inequivalent chains of edge-sharing VO5 square pyramids bridged by PO4 double tetrahedra. No structural phase transition has been observed in the temperature range between 300 and 120 K.

Polarized Raman and infrared vibrational analysis of (VO)2P2O7 single crystals

Abstract Single crystals of the low dimensional spin system compound (VO)2P2O7 were studied by polarized Raman spectroscopy and FT-IR spectroscopy in the spectral range 100– 1400 cm −1 . Characteristic groups of modes were compared with literature data of structurally related compounds. Though some differences to vibrational spectra of the referred compounds were detected, we achieved a satisfactory assignment of most of the high-energy modes to vibrations of the P2O7 polyhedra and the VO5 pyramids. In particular, we report the observation of an unusual softening of the prominent V=O vibration which we explain by the interlayer interaction of neighboring V and O atoms—an effect, which has been observed in NaV2O5, as well.

Oxygen stoichiometry and the problem of the growth of (VO)2P2O7 single crystals

Abstract The crystal growth of (VO) 2 P 2 O 7 single crystals is discussed with respect to the formation of a high-temperature nonstoichiometric phase (VO) 2 P 2 O 7+x . The interaction between vanadyl pyrophosphate and oxygen was investigated in the temperature region 600–900°C. It was found that the decay of the high-temperature nonstoichiometric phase at lower temperature led to imperfections in crystals grown in an atmosphere with oxygen concentration higher than 0.2%. Two mechanisms for the decay are considered. The oxygen content of the crystals may be reduced at the surface by evolution of gaseous oxygen, whereas in the bulk the formation of an oxygen-enriched solid phase is observed.

Thermal conductivity of (VO)2P2O7 single crystals

Thermal conductivity ϰ of single-crystal (VO)2P2O7 has been studied within the 4–300 K range. A break was found in the ϰ(T) relation about 200 K, in the region of the transition from diffuse antiferromagnetic ordering (200–4 K) to a classical paramagnet (T=200–300 K). In the low-temperature domain (4–200 K), one may expect an additional contribution to ϰ(T) from the magnon component of thermal conductivity.