An indexing program, TREOR, mainly based on trial-anderror methods is described. The program contains separate routines for cubic, tetragonal, hexagonal, orthorhombic, monoclinic and triclinic symmetries. Ten years usage has been analysed to improve the original program. For monoclinic indexing a specific short-axis test has been developed. The over-all success rate of the program has been found to be better than 90%, and considerably more for orthorhombic and higher symmetries. Computer Program AbstractsJ. AppL CrysL (1985). 18, 370The category Computer Program Abstracts provides a rapid means of communicating up-to-date information concerning both new programs or systems and significant updates to existing ones. Following normal submission, a Computer Program Abstract will be reviewed by one or two members of the IUCr Commission on Crystallographic Computing. It should not exceed 500 words in length and should use the standard format given on page 189 of the June 1985

Indexing of a powder diffraction pattern is still a critical point in procedures aiming at solving crystal structures from powder data. New code has been associated to the program TREOR90 in order to define an efficient peak search procedure, to modify the crystallographic decisions coded into TREOR90 to make it more exhaustive, to refine the selected unit cell automatically, and to make the entire procedure user friendly, via a graphical interface. The new program, called N‐TREOR, has been integrated into the package EXPO to create a suite of programs able to provide a structural model from the analysis of the experimental pattern. N‐TREOR is also available as a stand‐alone program.

A high-pressure phase of vanadium pentoxide, denoted beta-V2O5, has been prepared at P = 6.0 GPa and T = 1073 K. The crystal structure of beta-V2O5 has been studied by X-ray and neutron powder diffraction, and high-resolution transmission electron microscopy. The V atoms are six-coordinated within distorted VO6 octahedra. The structure is built up of quadruple units of edge-sharing VO6 octahedra linked by sharing edges along [010] and mutually connected by sharing corners along [001]. This arrangement forms layers of V4O10 composition in planes parallel to (100). The layers are mutually held together by weak forces. beta-V2O5 is metastable and transforms to alpha-V2O5 at 643-653 K under ambient pressure. Structural relationships between beta- and alpha-V2O5, and between beta-V2O5 and B-Ta2O5-type structures are discussed. The high-pressure beta-V2O5 layer structure can be considered as the parent of a new series of vanadium oxide bronzes with cations intercalated between the layers.

Powder pattern indexing can still be a challenge, despite the great recent advances in theoretical approaches, computer speed and experimental devices. More plausible unit cells, belonging to different crystal systems, are frequently found by the indexing programs, and recognition of the correct one may not be trivial. The task is, however, of extreme importance: in case of failure a lot of effort and computing time may be wasted. The classical figures of merit for estimating the unit‐cell reliability {i.e.M20 [de Wolff (1968). J. Appl. Cryst.1, 108–113] and FN [Smith & Snyder (1979). J. Appl. Cryst.12, 60–65]} sometimes fail. For this reason, a new figure of merit has been introduced in N‐TREOR09, the updated version of the indexing package N‐TREOR [Altomare, Giacovazzo, Guagliardi, Moliterni, Rizzi & Werner (2000). J. Appl. Cryst. 33, 1180–1186], combining the information supplied by M20 with additional parameters such as the number of unindexed lines, the degree of overlap in the pattern (the so‐called number of statistically independent observations), the symmetry deriving from the automatic evaluation of the extinction group, and the agreement between the calculated and observed profiles. The use of the new parameters requires a dramatic modification of the procedures used worldwide: in the approach presented here, extinction symbol and unit‐cell determination are simultaneously estimated. N‐TREOR09 benefits also from an improved indexing procedure in the triclinic system and has been integrated into EXPO2009, the updated version of EXPO2004 [Altomare, Caliandro, Camalli, Cuocci, Giacovazzo, Moliterni & Rizzi (2004). J. Appl. Cryst. 37, 1025–1028]. The application of the new procedure to a large set of test structures is described.

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