G. Aurelio

Atomic ordering and systematics of bonding lengths in the Ti–V omega phase: a neutron diffraction study

Abstract A new model describing the structural and bonding properties of the omega (Ω) phase in Zr–Nb alloys has recently been presented [12] . This model, which was aimed at explaining the composition dependence of the bonding lengths, predicts that the Ω phase is ordered, i.e., that some crystallographic sites are preferentially occupied by Zr atoms. Such feature, which should in principle be observed in other, related Ω phases, has not yet been tested against direct measurements. This problem has now been studied in the Ti–V system, which is the analogue of Zr–Nb in the 3d-transition series. Neutron diffraction measurements have been performed in quenched Ti–V alloys with V contents between 14 and 17 at.%. The diffraction spectra have been analysed using the Rietveld method, and a systematic analysis is reported here of the possibility of deviations from the random occupation of the two sublattices which are distinguished in the Ω structure. In addition, these new diffraction data are used in an evaluation of the shortest interatomic distances which are relevant for a comparison with the predictions of the model of Grad et al.

Interatomic distances in the stable and metastable bcc and omega structures of the transition metals : analysis of experimental and theoretical trends and correlations with Pauling's bond lengths

Abstract The name omega (Ω) phase refers to a high-pressure modification of the transition metals (TMs) Ti, Zr, and Hf. In alloys of Ti, Zr and Hf with other TMs, the Ω phase can be formed and retained metastably at room temperature by quenching the bcc structure, which is usually the stable high-temperature phase in these alloy systems. As a part of a long-term research program on the structural and bonding properties of TM phases, we present in this paper a detailed analysis of the shortest interatomic distances (SIDs) of the stable and metastable bcc and Ω structures of the 3d, 4d and 5d TMs, as well as the connections between these and the single-bond length D (1) entering into Pauling’s classical bond length vs. bond number relation (BLBNR). The analysis involves an extensive database including experimental data, very recent ab initio calculations, and new predictions based on the BLBNR for the bcc phase of the TMs. Our results lend support to early suggestions by Jamieson of the existence of a correlation between D (1) and the SID of the Ω phase of Ti and Zr. We also find that, for most TMs, D (1) and the SID of the Ω structure differ by less than about 2%, which implies that the Jamieson correlation might be considered as a reasonably accurate first approximation in estimating IDs and lattice parameters for unknown Ω phases of the TMs.

Phenomenological bond length-bond number correlations for Ti, Zr and Hf: assessment and applications to metastable structures

Abstract This paper deals with (i) the establishment of Pauling’s bond length vs. bond number relation (BLBNR) for Ti, Zr and Hf, and (ii) the analysis of the connections between the parameters entering into the BLBNR and the experimental interatomic distances (IDs) in the known solid phases of these elements. Recently assessed ID data on the hcp structure, which is the stable phase at room temperature and atmospheric pressure (RTAP) as well as on the bcc and omega (Ω) structures, which are metastable at RTAP, are treated in terms of the BLBNR, and various approaches are tested for the evaluation of the univalent metallic bond-length parameter, D (1), of Ti, Zr and Hf. Next, a comparison is made with Pauling’s method, which is based on determining an optimum D (1) parameter. With these results, a new interpretation is presented of the so-called Jamieson correlation, which involves the possibility of a connection between D (1) and the shortest ID in the Ω phase. Finally, the BLBNR is applied to the metastable fcc structure of Ti, Zr and Hf, and a new evaluation is performed of the difference between metallic bond lengths for coordination numbers 12 ( D (12)) and 8 ( D (8)), which is compared with early generalisations on D (12)– D (8).

Structural properties of metastable phases in Zr-Nb alloys. III. The athermal Ω phase

Abstract By means of high-resolution neutron diffraction experiments, a new study of the lattice parameters and other structural properties as well as the mass fraction of the Ω phase formed by quenching Zr–Nb alloys, has been made. With the present results and those from the recent neutron diffraction work by Benites and co-workers (J. Alloys Comp. 299 (2000) 183; 302 (2000) 192) reported on papers I and II of this series, an analysis is performed of the composition dependence of various key aspects of the diffusionless (‘athermal’) Ω phase transition in the Zr–Nb system. In particular, we discuss the misfit between the Ω particles and the parent bcc phase, the hexagonal to trigonal symmetry change occurring in Ω and the effect of the latter upon the shortest interatomic distance in this phase. The picture emerging from the present study is also used to investigate the interatomic distance correlation previously reported by Grad et al. (Z. Metallkd. 87 (1996) 726).

Bond-length analysis of the omega structure in Ti, Zr, Hf and their alloys: experimental data, new correlations and implications for chemical bonding models

Abstract An analysis is performed of the experimental information on the key interatomic distances (IDs) of the AlB 2 type structure, the so-called omega (Ω) phase in Ti and Zr, as well as in Ti–V and Zr–Nb alloys. Various remarkable correlations are found between these IDs and standard measures of the atomic size, and with Pauling’s bond-lengths. These observations are discussed in the light of the phenomenological bonding pictures of the Ω structure, and with our recent ab initio calculations of the electronic structure of this phase.

Analysis of multiple scattering and multiphonon contributions in inelastic neutron scattering experiments

Abstract We present a method of analysis of inelastic neutron scattering (INS) experiments aiming at obtaining the density of phonon states in an absolute scale, as well as a reliable value of the mean-square displacement of the atoms. This method requires the measurement of the neutron total cross section of the sample as a function of energy, which provides a normalization condition for the INS experiment, as well as a value of the mean-square displacement. The method is applied in the case of an incoherent neutron scattering system, viz. the Ti–52wt.% Zr alloy. The applicability of this method to the study of metal alloys and other systems is discussed.

Structural and magnetic study of cation substitution in layered cobaltites Y(Ba,Ca,Sr)Co2O5.5

In this work we present a study of the structural properties of Y(Ba, Sr,Ca)Co2O5+δ layered cobaltites focussing on the role of cationic disorder and size effects by a partial substitution of Ba by the smaller cations Ca and Sr. We have synthesized the polycrystalline compounds Y(Ba1xCax)Co2O5.5 with x = 0, 0.05 and 0.1, and Y(Ba1ySry)Co2O5.5 with y = 0.05 and 0.1. We characterized the samples within the 5-570 K temperature range, using neutron diffraction, magnetic measurements and electrical resistivity experiments. Neutron powder diffraction (NPD) data were collected on the high-intensity two-axes diffractometer D20 and in the high-resolution powder diffractometer D2B. We have found that the alkaline earth doping strongly affects the magnetic properties of the parent compound YBaCo2O5.5. With the partial substitution of Ba with smaller cations as Ca and Sr, we could highlight the competition between the AFM order observed in various RBaCo2O5.5 cobaltites (R is a rare earth), and the ferrimagnetic phase which is generally observed in a narrow temperature range. For the Ca series, the sample with x = 0.05 shows a small fraction of AFM phase which seems to coexist with a ferrimagnetic one below T≈190 K, whereas for x = 0.1 the AFM order is completely lost. A structural distortion is observed in our NPD data, associated to the metal-insulator transition. Recent studies in other members of the cobaltites family indicate that the distortion is due to a charge delocalization. For the Sr series a tetragonal structure is stabilized by the addition of Sr, and the ferrimagnetic order is replaced by an AFM phase for the sample with y = 0.1. The magnetic phases show even a more complex behaviour than in the Ca series. In addition, data collected at high temperature indicate that samples are irreversibly transformed at ≈550 K, where an orthorhombic to tetragonal transition is followed by a loss of oxygen in the structure. Until now, each of these phenomena had been analyzed separately in different rare-earth layered cobaltites, some showing orthorhombic symmetry, others tetragonal symmetry, some studied at low temperature, others at high temperature. It is for the first time that we can put all the ingredients together and show that not only the rare-earth is playing a role in the physics of the layered cobaltites, but also the disorder and size effects in the Basite, highlighting the delicate balance between highly competing structures, both crystallographically and magnetically.