CONTRIBUTIONS ON THE THERMAL-BEHAVIOR OF ANHYDROUS PHOSPHATES .7. PREPARATION, STRUCTURE, AND THERMAL-BEHAVIOR OF ORTHORHOMBIC NBPO5-DELTA

By a chemical vapour transport in a temperature gradient (e.g. 1000-degrees --> 900-degrees-C) it is possible to prepare the already known forms of NbPO5 and a new phase as single crystals. Using I2 or NH4Cl as transport agents alpha-NbPO5 was obtained as colourless crystals, which enabled a refinement of the already known tetragonal crystal structure to R = 1.73%. The dark blue crystals of the up to now unknown orthorhombic phase could only be prepared under reducing conditions (by adding NbP). Their crystal structure is related to monoclinic beta-NbPO5 (single crystal investigation, space group Pna2(1); a = 11.2875 angstrom; b = 6.6296 angstrom; c = 5.2871 angstrom; Z = 4; R = 3.17%), but the crystals are deficient in oxygen according to NbPO5-delta recognizable by the dark blue colour. When NbPO5-delta (orthorhombic) is ground in an agate mortar NbPO5-delta (monoclinic) is formed, which has the already known structure of beta-NbPO5. By heating (340-degrees-C) a reversible transformation from NbPO5-delta (monoclinic) to NbPO5-delta (orthorhombic) takes place. This is isostructural to WPO5 and also to an orthorhombic modification of (colourless) NbPO5, which we also could observe above 340-degrees-C.