Nb Sublattice Research Articles

Defect related anomalous mobility of small polarons in dielectric oxides at the example of congruent lithium niobate

Polarons play a major role in the description of optical, electrical and dielectrical properties of several ferroelectric oxides. The motion of those particles occurs by elementary hops among the material lattice sites. In order to compute macroscopic transport parameters such as charge mobility, normal (i.e. Fickian) diffusion laws are generally assumed. In this paper we show that when defect states able to trap the polarons for long times are considered, significant deviations from the normal diffusion behaviour arise. As an example of this behavior, we consider here the case of lithium niobate (LN). This can be considered as a prototypical system, having a rich landscape of interacting polaron types and for which a significant wealth of information is available in literature. Our analysis considers the case of a stoichiometric, defect-free lithium niobate containing a certain concentration of small electron polarons hopping on regular Nb sites, and compares it to the material in congruent composition, which is generally found in real-life applications and which is characterized by a large concentration of antisite NbLi defects. While in the first case the charge carriers are free polarons hopping on a regular Nb sublattice, in the second case a fraction of polarons is trapped on antisite defects. Thus, in the congruent material, a range of different hopping possibilities arises, depending on the type of starting and destination sites. We develop a formalism encompassing all these microscopic processes in the framework of a switching diffusion model which can be well approximated by a mobile–immobile transport model providing explicit expressions for the polaron mobility. Finally, starting from the Marcus–Holstein’s model for the polaron hopping frequency we verify by means of a Monte Carlo approach the diffusion/mobility of the different polarons species showing that, while free polarons obey the laws for normal diffusion as expected, bound polarons follow an anomalous diffusion behaviour and that in the case of the congruent crystal where mixed free and bound polaron transport is involved, our expressions indeed provide a satisfactory description.

High-Entropy Lithium Niobate Nanocubes for Photocatalytic Water Splitting under Visible Light.

The vast compositional space available in high-entropy oxide semiconductors offers unique opportunities for electronic band structure engineering in an unprecedented large room. In this work, with wide band gap semiconductor lithium niobate (LiNbO3) as a model system, we show that the substitutional addition of high-entropy metal cation mixtures within the Nb sublattice can lead to the formation of a single-phase solid solution featuring a substantially narrowed band gap and intense broadband visible light absorption. The resulting high-entropy LiNbO3 [denoted as Li(HE)O3] crystallizes as well-faceted nanocubes; atomic-resolution imaging and elemental mapping via transmission electron microscopy unveil a distinct local chemical complexity and lattice distortion, characteristics of high-entropy stabilized solid solution phases. Because of the presence of high-entropy stabilized Co2+ dopants that serve as active catalytic sites, Li(HE)O3 nanocubes can accomplish the visible light-driven photocatalytic water splitting in an aqueous solution containing methanol as a sacrificial electron donor without the need of any additional co-catalysts.

Atomic diffusion and microstructural evolution at the Nb/TiAl heterogenous interfaces during annealing at elevated temperatures

The relevant phase transformations induced by atomic diffusion at the vicinity of Nb/TiAl heterogenous interface were systematically investigated by establishing a novel system of TiAl-Nb couple via metallurgical bonding of Nb block and γ-TiAl powders by hot pressing sintering and post-annealing treatments. As a result, the resultant diffusion layer, acting as adhesive region linking the Nb and TiAl substrates, thickened with the increase of annealing temperature and the prolongation of holding time, which could be well described by the developed power function. A series of intermediate phases initially nucleated at the Nb/TiAl interface and grew toward both substrates, which was the result of the exchange, partitioning and recombination of solute atoms in the light of vacancy-mediated substitutional diffusion pattern. The phase transformation path on the Nb side was β→η→η + σ→σ→β0→β0 + γn, while that on the TiAl side was γ→γss→γss+ α→(γ + α)+ (α + β0)→α + β0→β0 + γn. Then the disorder-order transition of α→α2 and β0→B2 occurred during cooling. The progress of phase transformation slowed down as the diffusion proceeded, which was attributed to the decrease in atomic substitution efficiency due to the reduction of atomic concentration gradient and the extension of migration distance. In such a case, the banded σba split into spike σsp along the direction of maximum concentration gradient, and then shrunk to isolated σis until completely dissolved in β0 regime, principally dominated by Ti occupying Nb sublattices. The lenticular γn existed mainly in the form of twins, resulting from the lattice reconstruction in accordance with the orientation relationship of 111γ//110β0 and 1̅10γ//11̅1β0 in the Al partitioning region during the spontaneous fluctuation of atomic concentration of β0 regime. These findings filled some fundamental gaps with respect to TiAl-Nb diffusion reaction system, and afforded profound insights into the development of high-performance TiAl/Nb composites with tailorable microstructure.

Mechanism of tungsten strengthening hydrogen transportation in Nb48Ti27Co25 hydrogen permeable alloy membrane

Substitution of Nb by W in Nb48Ti27Co25 dual phase hydrogen permeable alloy was carried out. The elemental W replacing Nb mainly dissolved into the primary body-centered cubic (bcc)-(Nb) phase to modify its composition and crystal lattice. The W alloying effect on hydrogen transportation was systematically discussed based on hydrogen chemical potentials and first principle calculations. The substitution of Nb by W reduced the hydrogen solubility, and led to an improved intrinsic hydrogen diffusion coefficient, especially at temperature below 573 K, which promoted the hydrogen permeation flux of Nb48Ti27Co25 alloy membrane at 523 K. According to the theoretical study based on density function theory of the Nb–W system, the reduced hydrogen solubility can be attributed to the weakened binding strength between hydrogen atoms and the bcc-(Nb) lattice and the reduced charge gained by hydrogen atoms after W substitution for Nb. In addition, the W occupying the Nb sublattice can change the diffusion path of hydrogen atoms in the bcc-(Nb) lattice, thereby reducing the diffusion energy barrier for high hydrogen diffusivity.

Synergistic effects of microalloying and pre-straining on enhanced nanoprecipitation and creep property of alumina-forming austenitic stainless steels

Non-shearable metal carbide (MC) nanoparticles are one of the most effective strengthening media for improving creep resistance of alloys at elevated temperatures. However, it is difficult to achieve sufficient homogeneous dispersion due to their large crystallographic difference with the matrix. Herein, we report a feasible method to form massive NbC nanoprecipitation in an aluminum-forming stainless steel via the combination of microalloying and pre-straining. The employment of pre-straining generated dislocations uniformly distributed inside the alloy matrix, which provides high-density heterogeneous nucleation sites and enables fast pipe diffusion channels for involved solutes, thus substantially enhancing the nucleation rate of the secondary NbC. Moreover, due to its stronger affinity with C and larger diffusivity as compared with Nb, Ti atoms were found to occupy the Nb sublattice of the NbC at the onset of precipitation. The formation of (Ti,Nb)C nuclei with decreased lattice misfit (from 20.8% to 19.1%) effectively reduced the critical nucleation energy barrier and thus enhanced the nucleation rate accordingly. With the synergistic effect of 0.1 wt% Ti microalloying and 10% pre-straining, massive precipitation of refined secondary (Ti,Nb)C nanosized particles (4.4 ± 0.7 nm) with a much increased volume fraction (almost 6.8%) was practically realized, remarkably reducing the steady-state creep rate and prolonging the creep-to-rupture lifetime to 2044 h at 1023 K and 120 MPa. The current findings provide a feasible strategy to disperse strong yet massive nanoparticles within metallic metrics, which may shed new insights into the development of advanced high-temperature alloys with much enhanced creep properties at no expense of oxidation resistance.

Nominal Effect of Mg Intercalation on the Superconducting Properties of 2H-NbSe2.

2H-NbSe2 is a phonon-mediated, Fermi-surface topology-dependent multiband superconductor with an incommensurate charge-density wave (CDW) that coexists at a local level with superconductivity. Usually, the intercalation in 2H-NbSe2 enriches the CDW, enhances the c-axis lattice parameter, and distorts the Fermi surface, which result in a decrease in the superconducting transition temperature (Tc). The rate of decrease of Tc depends on the electronic structure, size, valence, magnetic nature, and electronegativity of the intercalating species. Herein, we report an unusual effect of Mg intercalation on the superconductivity of 2H-MgxNbSe2 (x = 0.0, 0.02, 0.06, 0.08, 0.10, and 0.12) synthesized by a high-temperature solid-state reaction method. Unlike other s- and p-block elements/species as intercalants (Rb, Sn, Ga, and Al) that have a sharp detrimental effect on the Tc of 2H-NbSe2 within 1-5% of intercalation, Mg is found to be an exception. Upon Mg intercalation up to x = 0.06, no remarkable changes in Tc as compared to the parent 2H-NbSe2 (Tc ∼ 6.7 K) are observed, and further intercalation results in a small decrease in Tc (for x = 0.12, Tc = 6.2 K). From heat-capacity measurements, it is inferred that superconducting Mg-intercalated 2H-NbSe2 exhibits strong electron-phonon coupling. Electronic structure calculations on two s-block element intercalated compounds of formula M0.125NbSe2 (M = Mg, Rb) show that Rb s-, p-, and d-states completely overlap with the Nb d states, while the Mg s states lie in a low-energy region as compared to Nb d states, indicating a weak interaction between the intercalant and the Nb sublattice in Mg0.125NbSe2 as compared to Rb0.125NbSe2. These results suggest that the electronic states of the Nb network in 2H-NbSe2 are least altered with Mg intercalation, which could be one of the reasons for the minimal effect on the Tc with intercalation.

Exploring alloying effect on phase stability and mechanical properties ofγ″-Ni3Nb precipitates with first-principles calculations

The superior mechanical strength of Inconel 718 and similar Nb containing alloys mainly results from the ordered γ″-Ni3Nb precipitates. By employing first-principles calculations, we comprehensively investigate the effects of alloying elements (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Y, Zr, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, and Pt) on site preference, phase stability and mechanical properties of the γ″-Ni3Nb. The calculation results indicate that Sc, Ti, V, Y, Zr, Mo, Hf, Ta and W prefer to occupy the Nb sublattice, and others tend to occupy the Ni sublattice. Nb-sublattice and strong Ni-sublattice occupying elements can thermodynamically stabilize the γ″-Ni3Nb. Meanwhile, the introduction of W, Os, Ta or Mo is beneficial to the enhancement of the elastic moduli of the γ″-Ni3Nb. All studied phases present inherent ductile and strong elastic anisotropic. The alloying elements, except for Pd, Hf, Zr and Mn, will cause the increase of the Vickers hardness of the γ″-Ni3Nb. Furthermore, the electronic structures are analyzed to trace the influence mechanism of alloying elements on the stability and mechanical properties. This work provides insights into the improvement of stability and mechanical properties of Inconel 718 and the design of the γ″-strengthened novel alloys.

Molecular realizations of 3D Heisenberg magnet: Critical scaling

The critical properties of two related bimetallic compounds {[MII(H2O)2]2[NbIV(CN)8]·4H2O}n (M = Mn, Fe) are reported. The compounds exhibit the transition to a magnetically ordered phase at Tc = 43 K in the case of the Fe-based sample (FeNb), and at Tc = 50 K for the Mn-based sample (MnNb). They differ in relative orientation of the M and Nb sublattices, which is parallel, i.e. ferromagnetic, in FeNb and antiparallel, i.e. ferrimagnetic, in MnNb. Critical exponents β and γ, governing the temperature dependence of the spontaneous magnetization below Tc and the temperature dependence of the initial susceptibility above Tc, respectively, have been determined using the Kouvel-Fisher method. Critical exponent δ has been obtained through the analysis of the critical magnetization isotherm. Thus determined values of the critical exponents were verified by employing two independent forms of the scaling equation of state. All the magnetization data are demonstrated to collapse either onto two universal branches corresponding to T〈Tc and T〉Tc, respectively, or onto a single universal curve. The critical exponents allow to ascribe FeNb as well as MnNb to the universality class of the three-dimensional (3D) Heisenberg model.

Investigation of Li/Nb-sublattices in ion implanted LiNbO3 by RBS and NRA in channelling configuration

Differently oriented LiNbO3 crystals were implanted at room temperature with 1MeV iodine ions to fluences between 2×1013 and 1×1014cm−2, which cover the transition from a low damage level up to complete amorphisation. The aim of this work was to explore the use of nuclear reaction analysis (NRA) in combination with Rutherford backscattering spectrometry (RBS) in channelling configuration for studying the damage evolution as a function of the ion fluence in both the Li and Nb sublattice. Protons with energies between 1.4 and 1.6MeV and a standard RBS setup were used. Scattering events detected at low energies result from Rutherford backscattering of protons on Nb and O atoms. At high energies alpha particles are registered, which result from the nuclear reaction between protons and Li atoms. Along different low-index crystallographic directions channelling effects within both the RBS and NRA part of the spectra are observed. However, the strength of channeling within the NRA part depends on the crystallographic direction investigated. These effects are explained by the nature of ion-channelling with respect to the small atomic number of Li and is supported by calculations of minimum yields (ratio of scattering yield in aligned and random direction) applying the computer code DICADA. The consequence is that damage studies with NRA can be only performed in Z-direction of LiNbO3. In this case, the Li and Nb sublattice were found to be similarly damaged after 1MeV iodine implantation.

Tantalum and niobium diffusion in single crystalline lithium niobate

Abstract LiNbO3 and LiTaO3 are isomorphous and Nb and Ta have the same valence electron configuration and the same ionic radii. This suggests the use of Ta as a tracer to probe the self-diffusion of Nb in LiNbO3. The diffusion system consisted of a 20 nm layer of LiTaO3 sputter deposited on top of (i) a congruent LiNbO3 single crystal, i.e. (48.3 ± 0.1) mol% Li2O, and on top of (ii) a VTE processed LiNbO3 single crystal with nearly stoichiometric composition, i.e. (49.9 ± 0.1) mol% Li2O. The diffusion anneals (1000 °C ≤ T ≤ 1100 °C) were performed under a constant oxygen partial pressure of 200 mbar. From the resulting SIMS depth profiles of tantalum a constant diffusivity was extracted which can be assumed to reflect the niobium self-diffusivity in LiNbO3. For sub-stoichiometric LiNbO3 the joint discussion of this work and of literature data on the basis of the generally accepted defect model, 4[NbLi4•] = [VLi′], suggests Nb transport in the Li sublattice. For hyper-stoichiometric LiNbO3 the defect model 5 V Nb 5 ′ = L i i • is derived from the Li3NbO4/LiNbO3 solution reaction of the VTE process designed to obtain Li2O-rich LiNbO3 in accordance with the Li2O–Nb2O5 phase diagram. This model is supported by theoretical calculations of the defect formation energy. Interestingly, the migration enthalpy for the Li vacancy mediated transport of the anti-site defect NbLi4• in the Li sublattice of congruent, i.e. sub-stoichiometric, LiNbO3 is similar (within a ± 10% error) to the one derived for the Nb vacancy mediated transport of Nb in the Nb sublattice of hyper-stoichiometric LiNbO3, i.e. about 3 eV. The significant discrepancies between our results and some earlier literature data can be consistently rationalised if the experimental procedures of those studies are carefully analysed.

Prediction of the site ordering behaviours of elements in C15 NbCr2-based intermetallics by combining thermodynamic model with ab-initio calculation

The ordering behaviours of the transition metal elements in NbCr2-based alloys were predicted by combining thermodynamic model with ab-initio calculation. The predicted results show that for the ternary alloying elements in C15-type NbCr2-based alloy 64Cr–32Nb–4M (M = Cr, Hf, Mn, Mo, Nb, Ta, Ti, V, W or Zr), Mn atoms occupy only Cr sublattices (16d) and Hf, Ta, Ti and Zr atoms always prefer to occupy the Nb sublattices (8a). It is also found that the site preference of alloying elements Hf, Ta, Ti, Zr, or Mn are independent of the xCr/xNb (atomic ratio) and heat treatment temperature, while the site preferences of V, Mo or W depend on those. Present predicted results agree considerably with the available literatures, and some disagreements were also argued in this paper, i.e., in some literatures, alloying element V atoms always prefer to 16d sublattices and Ti atoms vary strongly with the alloy composition (xCr/xNb) and heat treatment temperature.

Magnetocaloric effect in M–pyrazole–[Nb(CN)8] (M = Ni, Mn) molecular compounds

We report a study of magnetocaloric effect (MCE) in cyanido-bridged {[MII(pyrazole)4]2[NbIV(CN)8]⋅4H2O}n molecular compounds where M = Ni, Mn, pyrazole = C3H4N2. The substances show a sharp phase transition to a long range magnetically ordered state, with ferromagnetic coupling between M and Nb sublattices in the case of the Ni-based sample 1 (Tc = 13.4 K) and ferrimagnetic coupling for the Mn-based sample 2 (Tc = 23.8 K). The magnetic entropy change ΔS due to applied field change ΔH as a function of temperature was determined by the magnetization and heat capacity measurements. The maximum value of ΔS at μ0ΔH = 5 T is 6.1 J mol−1 K−1 (5.9 J kg−1 K−1) for 1 at T = 14 K and 6.7 J mol−1 K−1 (6.5 J kg−1 K−1) for 2 at T = 25 K. MCE data at different applied fields have been presented as one universal curve, which confirms magnetic transitions in 1 and 2 to be of second order. The temperature dependences of the n exponent characterizing the dependence of ΔS on ΔH have been obtained. The n(Tc) values, consistent with the shape of the magnetization curves, pointed to the 3D Heisenberg behaviour for 2 and some anisotropy, probably of the XY type, for 1. The (H/Tc)2/3 dependence of the maximum entropy change has been tested in the ferrimagnetic Mn2–L–[Nb(CN)8] (L = C3H4N2, C4H4N2) series.

Ferroelectric phase transition in LiNbO3: Insights from molecular dynamics

Ab initio molecular dynamics within density functional theory is used for the first time to model the ferroelectric¿ paraelectric phase transition in LiNbO(3). Our calculations show that the structural phase transition is not an abrupt event, but rather a continuous process occurring over a range of about 100K and involving different ionic species at different temperatures. Because of the different behavior of the Li and Nb sublattices, the ferroelectric transition displays both displacive and order-disorder character.

Effect of Fe Doping by Thermal in-Diffusion on the Defect Structure of Lithium Niobate

In this work we investigate the iron incorporation in thermally diffused Fe doped LN, by combining two experimental techniques, i.e. micro-Raman spectroscopy and proton induced X rays emission. Our results point out that in substituting for Li, Fe ions induces a decrease of NbLi antisite defects and rearrangement of the Nb sublattice.

Structural changes underlying the diffuse dielectric response inAgNbO3

Structural differences in the so-called M polymorphs of ${\text{AgNbO}}_{3}$ were analyzed using combined high-resolution x-ray diffraction, neutron total scattering, electron diffraction, and x-ray absorption fine-structure measurements. These polymorphs all crystallize with $Pbcm$ symmetry and lattice parameters $\ensuremath{\surd}2{a}_{c}\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}2{a}_{c}\ifmmode\times\else\texttimes\fi{}4{a}_{c}$ (where ${a}_{c}\ensuremath{\approx}4\text{ }\text{\AA{}}$ corresponds to the lattice parameter of an ideal cubic perovskite) which are determined by a complex octahedral tilt system $({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{\ensuremath{-}})/({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{+})$ involving a sequence of two in-phase and two antiphase rotations around the $c$ axis. Our results revealed that, similar to ${\text{KNbO}}_{3}$, the Nb cations in ${\text{AgNbO}}_{3}$ exhibit local off-center displacements correlated along Nb-Nb-Nb chains. The displacements appear to be present even in the high-temperature ${\text{AgNbO}}_{3}$ polymorphs where the Nb cations, on average, reside on the ideal fixed-coordinate sites. The onset of the $({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{\ensuremath{-}})/({a}^{\ensuremath{-}}{b}^{\ensuremath{-}}{c}^{+})$ tilting in the M polymorphs lifts the symmetry restrictions on the Nb positions and promotes ordering of the local Nb displacements into a long-range antipolarlike array. This ordering preserves the average $Pbcm$ symmetry but is manifested in electron diffuse scattering and corroborated by other local-structure sensitive techniques. Structural states previously identified as the ${\text{M}}_{3}$ and ${\text{M}}_{2}$ phases represent different stages of displacive ordering rather than distinct thermodynamic phases. Rietveld refinements indicated intimate coupling between the displacive behavior on the oxygen, Nb, and Ag sublattices. The $Pbcm$ symmetry of the octahedral framework precludes a complete ordering of Nb displacements so that some positional disorder is retained. This partial disorder likely gives a source to the dielectric relaxation which, according to previous spectroscopic studies, is the origin of the diffuse dielectric response exhibited by M-type ${\text{AgNbO}}_{3}$ at $\ensuremath{\approx}250\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$.

Transmission Electron Microscopy of Fe2Nb Laves Phase with C14 structure in Fe-Nb-Ni Alloys

AbstractThe lattice structure of the C14 Fe2Nb Laves phase with Ni in solution in Fe-Nb-Ni ternary alloys was examined by transmission electron microscopy. Binary stoichiometric Fe2Nb (Fe-33.3 at.% Nb) exhibits a featureless morphology with a low dislocation density. A similar morphology was observed in stoichiometric Fe2Nb containing 20 at.% Ni and in binary Fe-rich Fe2Nb (Fe-27.5 at.% Nb). In contrast, many planar faults parallel to the basal plane of the C14 structure were observed in Fe-rich Fe2Nb with Ni in solution, and the fault density increases with increasing Ni content up to 33.1 at.%. The high resolution transmission electron microscope (HRTEM) analysis revealed that the planar faults are related to the local change in the stacking sequence of the three 36-nets (triple layer) of the C14 structure. These results suggest that the presence of both, the point defects (Fe sublattice sites occupied by Ni atoms) and the anti-site defects (Nb sublattice sites occupied by excess Fe atoms), facilitate the formation of the planar faults.

XANES Mn K edge in NaNbO3 based ceramics doped with Mn and Bi ions

Sodium niobate based ceramics doped with Mn atoms have been sintered from oxides. The Bi co-dopants were introduced to increase the solubility limit for Mn atoms. Two series of samples were heated at lowered air pressure to produce oxygen nonstoichiometry. X-ray Absorption Near Edge Structure (XANES) spectra at Mn K-edge were measured for samples with and without Bi co-dopants. Theoretical XANES spectra were calculated using the Real Space Multiple Scattering method (FEFF 8 code). The simulation showed that ceramics samples have Pbcm symmetry and all of them have Mn atoms located in a Nb sublattice.

Site occupancy preferences in the B2 ordered phase in Nb–rich Nb–Ti–Al alloys

The β-(Nb, Ti) solid solution phase in the Nb–Ti–Al system has recently been observed to extend to Al concentrations as high as 40 at.%. Five alloys with compositions (in at.%) of Nb– xTi–15Al, where x=17, 28.3 and 34, Nb–25Ti–25Al and Nb–24Ti–40Al were investigated. The β-transus temperatures, lattice parameters and β → B2 (β o) ordering temperatures are reported. Site occupancy preferences of the B2 lattice as a function of alloy composition were examined through the atom location by channeling enhanced microanalysis (ALCHEMI) technique with results presented as ordering tie lines. Ti substitution for Nb on Nb sublattice sites was observed in all of the alloys, with the strength of partitioning between Ti and Al atoms being dependent on alloy composition. A correlation between the partitioning behaviors of the atomic species and the Nb:Ti ratio of the alloy compositions was observed, along with changes in the lattice parameters and microhardness.

Study of structural differences between stoichiometric and congruent lithium niobate

The structural differences between stoichiometric and congruent (lithium deficient) lithium niobate single crystals were studied by RBS- and NRA-channeling as well as perturbed angular correlation (PAC) measurements. The 111Cd-PAC investigations point out that a second Li site can be detected in congruent material, while only one is present in stoichiometric. Channeling studies of different axes and the comparison of the results with computer simulations corroborated former indications that this additional lattice site can be attributed to the formation of ilmenite type stacking faults. A comparative study of the energy dependence of the dechanneling showed that a remarkable disorder is also present in the Nb sublattice of the congruent crystals and that these defects have a point-like character.

Thermodynamic properties of NbH 0.84

The NbH 0.84 interstitial solid solution was extensively investigated by means of specific heat measurements in the temperature range 65–420 K and several first-order phase transitionsj λ′ → λ → β → α′ were observed at T pt ≊127, 227 and 390 K , respectively. The fc orthorhombic structure of the Nb sublattice was found by means of X-ray powder diffraction for all phases existing in the range 15–300 K. A negative Grüneisen parameter, γ G, was observed for the λ′ phase. Jumps of the entropy, ΔS, and of the molar volume, ΔV, were evaluated for the low-temperature transitions and discussed in terms of the Clausius-Clapeyron equation. The specific heat in the regions of existence of particular phases was analyzed in the framework of a simplified model containing lattice and electronic components and taking into account the diffusion of hydrogen atoms.