The crystal and magnetic structure of RCrO 4 oxides (R=Nd, Er and Tm) has been studied by owder neutron diffraction. These compounds crystallize with the zircon-type structure, showing tetragonal symmetry, space group I4 1 =amd: In the case of NdCrO 4 , magnetic susceptibility measurements reveal the existence of an antiferromagnetic ordering in which both Cr 5+ and Nd 3+ sublattices are involved. This ordering has been explained on the basis of a propagation vector k ¼ 0 and a collinear structure, described by the symmetry mode A x ; the ordered magnetic moments being 0.62 and 0.66 m B at 2 K for Nd 3+ and Cr 5+ , respectively. Magnetic susceptibility and magnetization measurements reveal that both ErCrO 4 and TmCrO 4 behave as ferromagnetic compounds with a Curie temperature of 15 and 18 K, respectively. Rietveld refinement of the neutron diffraction data for ErCrO 4 yields a collinear magnetic structure described with an F x mode. In the case of the TmCrO 4 oxide, the ferromagnetic sublattices of Tm 3+ and Cr 5+ are aligned antiparallel in the a2b plane, while along the c-axis the magnetic moments point to the same direction. In both compounds, the rather small values obtained for the Er 3+ and Tm 3+ ordered moments compared with the theoretical ones have been attributed to crystal field effects. The differences in the ferromagnetic structure of these compounds have been explained as the result of the higher rare-earth anisotropy of Tm 3+ when compared with Er 3+ , for which no magnetic component is present along the c-direction. r

This paper reports the specific conditions used in the preparation of RCrO oxides, R5Nd, Sm, Eu and Lu, the structural determination, 4 and the study of their magnetic behaviour. The structure of these compounds has been refined from X-ray powder diffraction by the Rietveld method, assigning the zircon type, space group I4 /amd. A linear decrease has been observed in the lattice parameters from ntiferromagnetic ordering in which both Cr and R sublattices are involved. In the case of NdCrO , the estimated Neel temperature 4 appears to be lower than 2 K; for the remaining oxides, it is 14.9 K, 15.9 K and 9.9 K for Sm, Eu and Lu, respectively. A superexchange mechanism has been proposed to explain such a magnetic behaviour. The pathways through which these interactions take place have also 51 been analysed, taking into account the structural features that these oxides present. The Cr plays an important role as a promoter of 31 these interactions in the R sublattice.

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RCrO 4 oxides (R=Pr, Gd, Tb, Tm, and Yb) have been synthesized at 773 K using the corresponding nitrates as precursors. X-ray diffraction data reveal that these samples are single phases and crystallize with the zircon-type structure, showing tetragonal symmetry, space group I4 1 /amd. All the compounds are antiferromagnetic and the Ne´el temperature, which depends on the R 3+ ion, takes values lower than 30 K. The presence of a canting appears to be responsible for the negative values of the magnetic susceptibility found below the compensation temperature. This uncommon phenomenon is named reversal of magnetization. It is field-dependent, being suppressed at 500 Oe for the TmCrO 4 compound. The highest value of the compensation temperature (24 K) corresponds to the YbCrO 4 oxide. A metamagnetic transition has been observed in all cases at critical fields ranging from 225 Oe (GdCrO 4 ) to 1600 Oe (YbCrO 4 ). # 2002 Elsevier Science (USA)

VCrSbO, has been prepared and its magnetic and electronic properties investigated. The crystal structure was refined from X-ray powder diffraction by the Rietveld method. The unit cell is tetragonal (space group P4,/rnnm, Z=2), a=4.5708(4) A and c=3.0281(3) A. The compound is isostructural with the rutile phase FeNbO, and stoichiometric with respect to all three constituent metals atoms. The electron diffraction pattern is consistent with a tetragonal symmetry.

PrCrO has been synthesized as a single phase using oxidizing conditions, with a strong control on temperature and time. The 4 zircon-type structure that this compound adopts has been refined from X-ray powder diffraction by the Rietveld method, showingt etragonal symmetry with space group I4 /amd, lattice parameters a57.341 (7) below 12 K. When the temperature decreases, the superexchange Pr -O-Cr -O-Pr antiferromagnetic interactions becomé predominant, giving rise to a net maximum at lower temperatures. The estimated Neel temperature is field-dependent, taking the value of 9 K when the magnetic field strength is 50 Oe. The M vs. H plots, at 2 and 8 K, indicate the existence of a metamagnetic transition, having obtained a critical field as low as 399 and 149 Oe respectively at the temperatures mentioned above.

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The magnetic properties of the YbCrO 4 oxide have been studied by both bulk magnetic measurements and 170 Yb M .ossbauer spectroscopy. Ferrimagnetic ordering is shown to be present below 25 K, driven by the exchange within the chromium sublattice. In the saturation state, the magnetic moment is 0.55 m B for Yb 3+ , and presumably 1 m B for Cr 5+ , the two sublattices being antiferromagnetically coupled. Rare earth chromates constitute a subgroup of the family of compounds of general formula RXO 4 , where R(III)=rare earth, while X(V) symbolises the following ions: V(V), As(V), P(V) and Cr(V) [1]. Few studies have been performed so far in the mentioned chromates [2]. We have focused on the magnetic properties of the zircon-type YbCrO 4 oxide. Basing on the existing studies on the isomorphic YbVO 4 , YbAsO 4 and YbPO 4 compounds, the influence of the Cr 5+ ion on the overall magnetic properties of the YbCrO 4 can be thoroughly assessed.In a first stage, we measured the magnetic susceptibility of the sample as a function of temperature to determine its magnetic ordering temperature, turning out to be 25 K. Subsequently, the magnetisation was recorded as a function of the external magnetic field up to 1 T at different temperatures. The M . ossbauer spectra on the 170 Yb isotope were measured between 4.2 and 30 K. Some selected spectra are represented in Fig. 2. An extra absorption has been detected near the centre of the spectrum coming from an impurity phase, the Yb 2 O 3 oxide, whose known spectrum has been included in Fig. 2 as a dashed line. Its substantially high Debye temperature, when compared with the one corresponding to the YbCrO 4 compound, can probably be the cause for its seemingly large M .ossbauer percentage, which amounts to 35(10)%.The YbCrO 4 M . ossbauer spectra comprise only a quadrupolar contribution between 25 and 30 K, with a quadrupolar coupling constant: a Q =3.5 mm/s. Below 25 K, a mixed magnetic-quadrupolar hyperfine pattern is clearly present, thus supporting the previously ARTICLE IN PRESS