J. Isasi

Synthesis, structure and conductivity study of new monovalent phosphates with the langbeinite structure

Abstract This paper reports the synthesis, structure and the conductivity study of the new monovalent phosphates of the formula Na 2 MTi(PO 4 ) 3 , M=Fe or Cr. The structures of these compounds have been refined from X-ray powder diffraction by the Rietveld method. Their reflections are indexed in the cubic system with langbeinite-type structure in the P2 1 3 space group. This structure can be best described in terms of [M 2 P 3 O 18 ] structural units composed of two MO 6 octahedra linked to each other through three PO 4 tetrahedra. Curie-type behaviour is observed in the magnetic susceptibility, which confirms the trivalent state of M metals and suggest no magnetic interactions. The electrical behaviour of these phases show that the activation energies are relatively high and they could be related to the previous structural analysis

Structural and magnetic characterization of RCrO4 oxides (R=Nd, Er and Tm)

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 = 0 and a collinear structure, described by the symmetry mode A x , the ordered magnetic moments being 0.62 and 0.66 μ 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 a-b 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.

Synthesis, structural characterization and magnetic properties of RCrO4 oxides, R=Nd, Sm, Eu and Lu

Abstract This paper reports the specific conditions used in the preparation of RCrO 4 oxides, R=Nd, Sm, Eu and Lu, the structural determination, 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 1 /amd . A linear decrease has been observed in the lattice parameters from NdCrO 4 to LuCrO 4 , according to the lanthanide contraction. Magnetic susceptibility measurements reveal the existence of an antiferromagnetic ordering in which both Cr 5+ and R 3+ sublattices are involved. In the case of NdCrO 4 , the estimated Neel temperature 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 been analysed, taking into account the structural features that these oxides present. The Cr 5+ plays an important role as a promoter of these interactions in the R 3+ sublattice.

Ionic conductivity of a new defect pyrochlore

Abstract The synthesis and crystal structure of the new oxide Rb 3 CrTe 3 O 12 are reported. This compound can be considered as derived from a cubic pyrochlore (A 2 B 2 O 7 ) of formula Rb 3 2 Cr 1 2 Te 3 2 O 6 in which both A-sites (Rb) and anionic ones are partially occupied (A 1.5 B 2 O 6 ). This kind of structure enables the mobility of Rb + cations through well defined 3D-tunnels. Electrical conductivity measurements agree with this assumption and the experimental data are interpreted on these bases.

Synthesis, structure determination and magnetic susceptibilities of the oxides Ln3Li5Sb2O12 (Ln ≠ Pr, Nd, Sm)

Abstract The oxides Ln 3 Li 6 Sb 2 O 12 (Ln≠Pr, Nd, Sm) were prepared in air by heating a mixture of Ln 2 O 3 , LiNO 3 and Sb 2 O 3 in the temperature range 1023–1243 K. Lattice parameters as well as atomic coordinates in the space group 12,3 (Z≠8) are established from X-ray powder diffraction data by the Rietveld method. Magnetic susceptibilities from 4.2 to 300 K follow a Curie-Weiss law above 50 K (Ln≠Pr) or 70 K (Ln≠Nd). This is attributed to the splitting of the ground state associated with the Ln 3+ ions by the influence of the crystal field. The magnetic moments, 3.54 and 3.61 μ D for praseodymium and neodymium respectively, agree with those calculated by Hunds formula. The Sm 3+ behaviour can be explained by taking into account that the splitting of the multiplets is not too large compared to kT. The magnetic moment observed at room temperature for this cation is 1.6 μ B .

Synthesis and structure determination of a new phase in the La2O3Li2OSb2O5 system

Abstract The new phase La3Li5Sb2O12, isostructural with the Nb(V) and Ta(V) compounds of the same stoichiometry, has been isolated and characterized. The crystal structure is refined from X-ray powder diffraction data by Rietvelds method. The unit cell is cubic (space group I213, Z = 8) with a = 12.8460(5) A and the final reability factors are: RP = 14.35, Rb = 8.93.

New MM′O4 oxides derived from the rutile type: synthesis, structure and study of magnetic and electronic properties

The synthesis, structural characterization and the variation of magnetic and electronic properties of MM′O4 (M=Rh, Ni, Co; M′=Sb, V, Te) oxides are reported. These compounds can be regarded as being derived from TiO2 rutile type, when Ti atoms are formally substituted by two kinds of cations. The crystal structures were refined from X-ray powder diffraction by the Rictveld method. Different structural types are found for all these materials. Magnetic susceptibility results and electrical conductivity measurements are discussed in connection with the structural model proposed in each case.

Solid solutions with rutile structure : electronic behaviour and ultraviolet photoelectron spectrometry study

Solid solutions of composition Ti2.1M0.3(Sb1−xNbx)0.6O6 (M=Co, Ni and Cu) in the range 0≤ x≤ 1 are obtained by thermal treatment of the appropriate amounts of simple oxides. The crystal structures have been determined by employing Rietveld structure analysis of powder X-ray diffraction data. Electrical conductivity, magnetic and ultraviolet photoelectron spectrometry (UPS) measurements for these phases are also discussed.

Magnetic behavior of ErCrO4 oxide

Abstract ErCrO 4 has been synthesized as a single phase by the nitrate precursor method under an oxygen flow at 753 K. The structure of this compound has been refined from X-ray and neutron powder diffraction data, showing the zircon-type structure, S.G. I4 1 /amd and lattice parameters a =7.062(1) A, c =6.201(3) A. Magnetic susceptibility and magnetization measurements reveal the existence of incipient ferromagnetic interactions in the Cr 5+ sublattice below 25 K. When the temperature decreases, the superexchange Er 3+ –O–Cr 5+ –O–Er 3+ antiferromagnetic interactions become predominant. The estimated Neel temperature is 15 K. A metamagnetic transition has been detected at the critical field of 375 Oe. Neutron diffraction data show a three-dimensional antiferromagnetic ordering in which both Cr 5+ and Er 3+ sublattices are involved. Such ordering takes place with a propagation vector k =[0].

A combined study of the magnetic properties of GdCrO4

We have performed a detailed study of the magnetic properties of GdCrO4 at low temperatures by complementary use of different macroscopic and microscopic physical techniques. A ferromagnetic order is established in this oxide below TC = 22 K. The ordered magnetic moments of the Cr5+ ions are located along the crystallographic c-axis, forming an angle of ≈24° with the ordered moments of the Gd3+ ions. Surprisingly, only 20% of the Gd3+ sublattice orders at the Curie temperature of 22 K, while the remaining 80% stays paramagnetic down to around 10 K. This 80% of the Gd3+ ions may be ascribed to a low-temperature orthorhombic phase. Moreover, the Gd3+ magnetic order seems to be triggered by a relatively large transferred hyperfine field coming from the Cr5+ sublattice.