Strong Anharmonicity Research Articles

Tunable anharmonicity versus high-performance thermoelectrics and permeation in multilayer (GaN)1–x (ZnO) x

Nonisovalent (GaN)1−x (ZnO) x alloys are more technologically promising than their binary counterparts because of the abruptly reduced band gap. Unfortunately, the lack of two-dimensional (2D) configurations as well as complete stoichiometries hinders to further explore the thermal transport, thermoelectrics, and adsorption/permeation. We identify that multilayer (GaN)1−x (ZnO) x stabilize as wurtzite-like Pm-(GaN)3(ZnO)1, Pmc21-(GaN)1(ZnO)1, P3m1-(GaN)1(ZnO)2, and haeckelite C2/m-(GaN)1(ZnO)3 via structural searches. P3m1-(GaN)1(ZnO)2 shares the excellent thermoelectrics with the figure of merit ZT as high as 3.08 at 900 K for the p-type doping due to the ultralow lattice thermal conductivity, which mainly arises from the strong anharmonicity by the interlayer asymmetrical charge distributions. The p–d coupling is prohibited from the group theory in C2/m-(GaN)1(ZnO)3, which thereby results in the anomalous band structure versus ZnO composition. To unveil the adsorption/permeation of H+, Na+, and OH− ions in AA-stacking configurations, the potential wells and barriers are explored from the Coulomb interaction and the ionic size. Our work is helpful in experimental fabrication of novel optoelectronic and thermoelectric devices by 2D (GaN)1−x (ZnO) x alloys.

Intrinsic Ultralow Lattice Thermal Conductivity in the Full-Heusler Compound Ba2AgSb

Full-Heusler thermoelectric materials have intrinsically low lattice thermal conductivity. Our first-principles calculations show that ${\mathrm{Ba}}_{2}\mathrm{Ag}\mathrm{Sb}$ is a semiconductor with an indirect band gap of 0.49 eV. The electronic band degeneracy and pockets near the Fermi level facilitate electron transport. The short phonon relaxation time, small group velocity (1.89 km ${\mathrm{s}}^{\ensuremath{-}1}$), and large phonon scattering space reflect the intense phonon-phonon scattering. The large Gr\"uneisen parameter (1.44) accounts for the strong phonon anharmonicity, thus the low lattice thermal conductivity of $0.5\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 800 K. The isotropic figure of merit with a maximum value of 4.7 at 750 K is comparable to that of reported materials. The distribution of phonon momentum uncovers the important role of $\mathrm{Ag}$ in resisting thermal transport. The analysis of symmetry-based phonon-phonon scattering routes reveals the significance of symmetry on phonon scattering. The crystal structure of ${\mathrm{Ba}}_{2}\mathrm{Ag}\mathrm{Sb}$ can be used to regulate chemical elements to build high-performance thermoelectric materials. Our calculations provide an effective way to design thermoelectric materials, stimulating the study of full-Heusler materials.

Anomalous Thermoelectric Performance in Asymmetric Dirac Semimetal BaAgBi.

Multiple-band degeneracy has been widely recognized to be beneficial for high thermoelectric performance. Here, we discover that the p-type Dirac bands with lower degeneracy synergistically produce a higher Seebeck coefficient and electrical conductivity in topological semimetal BaAgBi. The anomalous transport phenomenon intrinsically originated from the asymmetric electronic structures: (i) complete p-type Dirac bands near the Fermi level facilitate high and strong energy-dependent hole relaxation time; (ii) the presence of additional parabolic conduction valleys allows for a large density of states to accept scattered electrons, leading to an enlarged hole-electron relaxation time ratio and, thus, weakened bipolar effect. In combination with the strong lattice anharmonicity, an exceptional p-type average ZT of 0.42 is achieved from 300 to 600 K, which can be dramatically enhanced to 1.38 via breaking the C3v symmetry. This work uncovers the underlying mechanisms governing the abnormal transport behavior in Dirac semimetal BaAgBi and highlights the asymmetric electronic structures as target features to discover/design high-performance thermoelectric materials.

Enhanced thermoelectric performance by lone-pair electrons and bond anharmonicity in the two-dimensional Ge2Y2 family of materials with Y=N , P, As, or Sb

Using density functional theory combined with the Boltzmann transport equation, the charge, thermal transport, and thermoelectric properties in two-dimensional (2D) ${\mathrm{Ge}}_{2}{Y}_{2}$ ($Y=\text{N}$, P, As, or Sb) monolayers characterized by two structural phases, i.e., $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Y}}_{2}$ and $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Y}}_{2}$, have been studied systematically. Our theoretical results demonstrate that the lone-pair electrons have remarkable influences on their lattice thermal conductivity. By performing comparative studies on the two different structures of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}$, we uncover that the above influences not only originate from the interactions between the lone-pair electrons around Sb atoms and the bonding electrons of the adjacent Ge atom, but also from the interlayer Coulomb repulsive forces of lone-pair electrons distributed in different layers. The latter leads to a strong anharmonicity, which greatly suppresses the lattice thermal conductivity. Thus, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}$ monolayer has an ultralow thermal conductivity with 0.19 W/mK, while $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}$ monolayer with 5.1 W/mK at the temperature 300 K. Owing to the ultralow lattice thermal conductivity induced by lone-pair electrons, the predicted maximum value of the thermoelectric figure of merit ($ZT$) reaches 1.2 for $p$-type and 1.18 for $n$-type doping $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}$. Our theoretical results put forward another effective mechanism to design and optimize 2D thermoelectric materials with high thermoelectric conversion efficiency.

Magnetic Control of Soft Chiral Phonons in PbTe.

PbTe crystals have a soft transverse optical phonon mode in the terahertz frequency range, which is known to efficiently decay into heat-carrying acoustic phonons, resulting in anomalously low thermal conductivity. Here, we studied this phonon via polarization-dependent terahertz spectroscopy. We observed softening of this mode with decreasing temperature, indicative of incipient ferroelectricity, which we explain through a model including strong anharmonicity with a quartic displacement term. In magnetic fields up to 25T, the phonon mode splits into two modes with opposite handedness, exhibiting circular dichroism. Their frequencies display Zeeman splitting together with an overall diamagnetic shift with increasing magnetic field. Using a group-theoretical approach, we demonstrate that these observations are the result of magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. Thus, our Letter reveals a novel process of controlling phonon properties in a soft ionic lattice by a strong magnetic field.

A new salt-inclusion compound, |Ag4Br|@[B7O12], with a novel type of the porous double-layered borate anion and strong anharmonicity of the “guest” sublattice

A new triclinic layered borate, |Ag4Br|@[B7O12], has been prepared by glass crystallization and slow cooling stoichiometric melts in evacuated silica ampoules. The structure is based upon a layered borate anion 7:∞2 [(4: 2Δ + 2 T) + (3: 2Δ + T)] with an architecture yet not observed among inorganic borates. The layers consist of two parts comprised of tri- and tetraborate groups; it is ∼7 Å thick and oriented parallel to (001). The interlayer space is filled by the cationic [Ag4Br]3+sublattice exhibiting disorder and anharmonicity at the silver sites. The new compound was characterized by thermal analysis, high-temperature powder X-ray diffraction, vibrational and UV–Vis–NIR spectroscopy, as well as DFT calculations. For the triclinic unit cell, the eigenvalues of the thermal expansion tensor and the orientation of the thermal expansion tensor relative to the crystallographic axes are determined. The thermal expansion of the new borate bromide is strongly anisotropic, being marginal in the plane of the B–O layers (α11 = 8.2 (1), α22 = 4.71 (3) × 10−6 °С−1) while quite large in the normal direction (α33 = 38.2 (3) × 10−6).

Revisiting the phase transition sequence in L-methionine: Description of the disordering mechanism in an essential amino acid.

Raman spectroscopy investigations on L-methionine (L-Met) performed in a large temperature range (170-420K) and in a wide spectral window (5-3600cm-1) have revealed an extended disordering mechanism triggered by thermally activated motions of the terminal side-chain atoms, from 250 up to 390K. This very progressive disordering process is characterized by two thermodynamic features, the first corresponding to a broad endotherm (250 → 310K) marking the beginning of the process, while the second ending the disordering transformation is a sharp endothermic peak at 390K. These thermodynamic events are correlated with the softening of lattice vibrations and the increase of the quasielastic scattering, considered as the signatures of displacive phase transitions. The amorphous-like band-shape of the low-frequency Raman spectrum collected above 390K, resulting from the strong anharmonicity of local motions, is contrasting with the detection of additional Bragg peaks above 390K by x-ray diffraction, consistent with the Cp jump accompanying the endothermic peak. These observations suggest that L-Met is progressively dynamically disordered adopting additional configurations in the crystalline lattice through rotations of CH3 and the side-chain flexibility not clearly detected by x-ray diffraction. These results should be crucial for considering the stability of dried proteins composed of methionine residues.

Study on the thermoelectric properties of p-type doped CsCdF3 and CsHgF3 with quartic anharmonicity

The cubic perovskite materials CsCdF3 and CsHgF3 have extremely low lattice thermal conductivity, but due to the limitations of the previous calculation methods. The CsHgF3 have been considered unstable for a long time. Through self-consistent phonon theory and compressed sensing technology, we verified the stability of CsHgF3, and studied the thermoelectric transport characteristics of p-type doped CsCdF3 and CsHgF3 based on the first principles calculations combined with the Boltzmann transport equation. In addition, we also discussed the influence of the quartic anharmonicity on the lattice dynamics and thermal transport properties of CsHgF3. The results show that the halogen elements in CsHgF3 have strong quartic anharmonicity and directly affect the stability of the material. The lattice thermal conductivity of CsCdF3 and CsHgF3 calculated by self-consistent phonon theory are 2.03 W/mK and 1.23 W/mK at 300 K, respectively, which are higher than the results obtained by the traditional method, but still lower than most existing classical thermoelectric materials. By combining the obtained thermoelectric coefficients, the calculated thermoelectric figure of merit can reach 0.799 and 1.275 for CsCdF3 and CsHgF3 at 700 K, which shows that CsCdF3 and CsHgF3 have potential development prospects in high-temperature thermoelectric application field.

Thermoelectric Performance of the 2D Bi2Si2Te6 Semiconductor.

Bi2Si2Te6, a 2D compound, is a direct band gap semiconductor with an optical band gap of ∼0.25 eV, and is a promising thermoelectric material. Single-phase Bi2Si2Te6 is prepared by a scalable ball-milling and annealing process, and the highly densified polycrystalline samples are prepared by spark plasma sintering. Bi2Si2Te6 shows a p-type semiconductor transport behavior and exhibits an intrinsically low lattice thermal conductivity of ∼0.48 W m-1 K-1 (cross-plane) at 573 K. The first-principles density functional theory calculations indicate that such low lattice thermal conductivity is derived from the interactions between acoustic phonons and low-lying optical phonons, local vibrations of Bi, the low Debye temperature, and strong anharmonicity result from the unique 2D crystal structure and metavalent bonding of Bi2Si2Te6. The Bi2Si2Te6 exhibits an optimal figure of merit ZT of ∼0.51 at 623 K, which can be further enhanced by the substitution of Bi with Pb. Pb doping leads to a large increase in power factor S2σ, from ∼3.9 μW cm-1 K-2 of Bi2Si2Te6 to ∼8.0 μW cm-1 K-2 of Bi1.98Pb0.02Si2Te6 at 773 K, owing to the increase in carrier concentration. Moreover, Pb doping induces a further reduction in the lattice thermal conductivity to ∼0.38 W m-1 K-1 (cross-plane) at 623 K in Bi1.98Pb0.02Si2Te6, due to strengthened point defect (PbBi') scattering. The simultaneous optimization of the power factor and lattice thermal conductivity achieves a peak ZT of ∼0.90 at 723 K and a high average ZT of ∼0.66 at 400-773 K in Bi1.98Pb0.02Si2Te6.

Thermodynamics Modeling for Actinide Monocarbides and Mononitrides from First Principles

The high-temperature thermodynamical properties for the actinide monocarbides and mononitrides ThC, ThN, UC, UN, PuC, and PuN are calculated from first-principles electronic-structure theory. The electronic structure is modeled with density-functional theory (DFT) and is fully relativistic, including the spin-orbit interaction. Furthermore, the DFT is extended to account for orbital–orbital interactions, by means of a parameter-free orbital-polarization (OP) technique, that has proven to be essential for the 5f electrons in plutonium. Strong anharmonicity and the temperature dependence of the lattice vibrations are captured with the self-consistent ab initio lattice dynamics (SCAILD) method. The calculated free energies and heat capacities are compared to published results from quasi-harmonic (QH) theory, and experiments, where available. For the uranium and plutonium compounds, we make use of CALPHAD assessments to help evaluate the theory. Generally, our anharmonic relativistic approach compares well with both CALPHAD and experiments. For the thorium compounds, our theory is in good accord with QH modeling of the free energy at lower temperatures but for the heat capacity the comparison is less favorable.

Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi2B2C

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. A broad spectral phonon line shape is often interpreted as a marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel sections of the Fermi surface connected by the phonon momentum. Alternatively broad phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of compounds.

Twofold rattling mode-induced ultralow thermal conductivity in vacancy-ordered double perovskite Cs2SnI6.

We report a first-principles study of lattice vibrations and thermal transport in Cs2SnI6, the vacancy-ordered double perovskite. Twofold rattlers of Cs atoms and SnI6 clusters in Cs2SnI6, being different from CsSnI3 with only Cs atom rattlers, largely scatter heat-carrying acoustic phonons strongly coupled with low-lying optical phonons and lower phonon group velocity. Using renormalized phonon dispersions at finite temperatures, we reveal that anharmonicity and twofold rattling modes induce an ultralow thermal conductivity at room temperature.

Significantly suppressed thermal transport by doping In and Al atoms in gallium nitride.

Thermal transport plays a key role in the working stability of gallium nitride (GaN) based optoelectronic devices, where doping has been widely employed for practical applications. However, it remains unclear how doping affects thermal transport. In this study, based on first-principles calculations, we studied the doping effect on the thermal transport properties of GaN by substituting Ga with In/Al atoms. The thermal conductivities at 300 K along the in-plane(out-of-plane) directions of In- and Al-doped GaN are calculated to be 7.3(8.62) and 12.45(11.80) W m-1 K-1, respectively, which are more than one order of magnitude lower compared to that of GaN [242(239) W m-1 K-1]. From the analysis of phonon transport properties, we find that the low phonon group velocity and small phonon relaxation time dominate the degenerated thermal conductivity, which originated from the strong phonon anharmonicity of In/Al-doped GaN. Furthermore, by examining the crystal structure and electronic properties, the lowered thermal conductivity is revealed lying in the strong polarization of In-N and Al-N bonds, which is due to the large difference in electronegativity of In/Al and N atoms. The results achieved in this study have guiding significance to the thermal transport design of GaN-based optoelectronic devices.

Low thermal conductivity and high performance anisotropic thermoelectric properties of XSe (X = Cu, Ag, Au) monolayers.

Combining density functional theory (DFT) and semi-classic Boltzmann transport theory, we report the thermoelectric (TE) performance of a family of two-dimensional (2D) group IB-selenides XSe (X = Cu, Ag, Au). The results show that these monolayers exhibit small and anisotropic phonon velocities (0.98-3.84 km s-1), large Grüneisen parameters (up to 100), and drastic phonon scattering between the optical and acoustic phonons. These intrinsic properties originate from strong phonon anharmonicity and suppress the heat transport capacity, resulting in low lattice thermal conductivities (12.54 and 1.22 W m-1 K-1) along the x- and y-directions for a CuSe monolayer. Among our studied monolayers, the 2D CuSe monolayer possesses the most remarkable TE performance with ultrahigh ZT (3.26) for n-type doping along the y-direction at 300 K. CuSe monolayer can achieve higher thermoelectric conversion efficiency at a lower synthetic preparation cost than the expensive AgSe and AuSe monolayers, and our work provides a theoretical basis for paving the way for further experimental studies.

Abnormal enhancement of thermal conductivity by planar structure: A comparative study of graphene-like materials

The traditional knowledge that the lone-pair electrons lead to low thermal conductivity provides important clues for the prediction of thermal conductivity and fundamental understanding on phonon transport. In this study, we reported a counterintuitive phenomenon that the thermal conductivity can increase by up to two orders of magnitude after activating the lone-pair electrons. Boron atoms are introduced to the two-dimensional (2D) nitrogene, blue phosphorene, arsenene, and antimonene to activate the lone-pair electrons, i.e., graphene-like boron nitride (g-BN), boron phosphide (g-BP), boron arsenide (g-BAs), and boron antimonide (g-BSb). This phenomenon is intuitively reflected in the competitive effect of phonon thermal transport (1) between in-plane and out-of-plane acoustic phonon modes, (2) between Grüneisen parameter and scattering phase space, and (3) between unbonded s-electrons pair and their space appearance. The essence of competition lies in the feedback adjustment of the structure to the electronic distribution, which reflects the high sensitivity of the thermal conductivity on the structure and overcomes the strong anharmonicity caused by the lone-pair electrons. Our study provides a counterexample to the traditional model of low thermal conductivity caused by lone-pair electrons. The results reported in this study provide a new perspective on the thermal transport, which is helpful to understand the key role of structure in phonon thermal transport in 2D materials.

Ultralow lattice thermal conductivity enables high thermoelectric performance in BaAg2Te2 alloys

Due to the intrinsic low lattice thermal conductivity (κL), AB2X2 Zintl compounds with a P3-M1 crystal structure have been demonstrated as potential thermoelectric candidates. Recently, a novel AB2X2 compound with a Pnma crystal structure has been revealed to possess an even lower κL, which motivates this work to focus on BaAg2Te2 for its potential for thermoelectric applications. Impressively, pristine BaAg2Te2 shows a κL lower than ∼0.3 W/m-K in the entire temperature range, being the lowest one ever reported in AB2X2 thermoelectrics. Theoretically, such a low κL comes from intrinsically strong anharmonicity and low sound velocity. With help of Cu-alloying at Ag site for increasing carrier concentration and for reducing κL, a peak zT up to ∼1.3 is achieved in BaAgCuTe2 alloy, and its single-leg device with Ag as the electrode enables an output power of ∼120 mW under a temperature difference of ∼375 K, demonstrating BaAg2Te2 based alloys as promising thermoelectric materials.

Low thermal conductivity and interface thermal conductance in SnS2

After the discovery of graphene, there have been tremendous efforts in exploring various layered two-dimensional (2D) materials for their potential applications in electronics, optoelectronics, as well as energy conversion and storage. One of such 2D materials, SnS2, which is earth abundant, low in toxicity, and cost effective, has been reported to show a high on/off current ratio, fast photodetection, and high optical absorption, thus making this material promising for device applications. Further, a few recent theoretical reports predict high electrical conductivity and Seebeck coefficient in its bulk counterparts. However, the thermal properties of SnS2 have not yet been properly explored, which are important to materialize many of its potential applications. Here, we report the thermal properties of SnS2 measured using the optothermal method and supported by density functional theory (DFT) calculations. Our experiments suggest very low in-plane lattice thermal conductivity (\k{appa} = 3.20 +- 0.57 W m-1 K-1) and cross-plane interfacial thermal conductance per unit area (g = 0.53 +- 0.09 MW m-2 K-1) for monolayer SnS2 supported on a SiO2/Si substrate. The thermal properties show a dependence on the thickness of the SnS2 flake. Based on the findings of our DFT calculations, the very low value of the lattice thermal conductivity can be attributed to low group velocity, a shorter lifetime of the phonons, and strong anharmonicity in the crystal. Materials with low thermal conductivity are important for thermoelectric applications as the thermoelectric power coefficient goes inversely with the thermal conductivity.

Thermal conductivity of CaSiO3 perovskite at lower mantle conditions

Thermal conductivity ($\kappa$) of mantle minerals is a fundamental property in geodynamic modeling. It controls the style of mantle convection and the time scale of the mantle and core cooling. Cubic CaSiO$_3$ perovskite (CaPv) is the third most abundant mineral in the lower mantle (LM) (7 vol%). However, despite its importance, no theoretical or experimental estimate of CaPv's $\kappa$ exists. Theoretical investigations of its properties are challenging because of its strong anharmonicity. Experimental measurements at relevant high pressures and temperatures are equally challenging. Here we present $ab$ $initio$ results for CaPv's $\kappa$ obtained using an established phonon quasiparticle approach that can address its strong anharmonicity. We also offer experimental measurements of $\kappa$. Predictions and measurements are in good agreement and reveal a surprisingly large $\kappa$ for cubic CaPv. Despite its relatively low abundance, CaPv's $\kappa$ might increase the lower mantle $\kappa$ by approximately 10%, if accounted for. $\kappa$ of mantle regions enriched in crust material will be more strongly impacted.

Achieving Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in GeTe Alloys via Introducing Cu2Te Nanocrystals and Resonant Level Doping.

The binary compound of GeTe emerging as a potential medium-temperature thermoelectric material has drawn a great deal of attention. Here, we achieve ultralow lattice thermal conductivity and high thermoelectric performance in In and a heavy content of Cu codoped GeTe thermoelectrics. In dopants improve the density of state near the surface of Femi of GeTe by introducing resonant levels, producing a sharp increase of the Seebeck coefficient. In and Cu codoping not only optimizes carrier concentration but also substantially increases carrier mobility to a high value of 87 cm2 V-1 s-1 due to the diminution of Ge vacancies. The enhanced Seebeck coefficient coupled with dramatically enhanced carrier mobility results in significant enhancement of PF in Ge1.04-x-yInxCuyTe series. Moreover, we introduce Cu2Te nanocrystals' secondary phase into GeTe by alloying a heavy content of Cu. Cu2Te nanocrystals and a high density of dislocations cause strong phonon scattering, significantly diminishing lattice thermal conductivity. The lattice thermal conductivity reduced as low as 0.31 W m-1 K-1 at 823 K, which is not only lower than the amorphous limit of GeTe but also competitive with those of thermoelectric materials with strong lattice anharmonicity or complex crystal structures. Consequently, a high ZT of 2.0 was achieved for Ge0.9In0.015Cu0.125Te by decoupling electron and phonon transport of GeTe. This work highlights the importance of phonon engineering in advancing high-performance GeTe thermoelectrics.

Isotopic dependence of the frequency of optical vibrations in molybdenum monohydride

It is currently known that three hydrides – PdHx, MoHx, and TiHx – exhibit an inverse isotope effect in superconductivity. Namely, the phase with a heavier hydrogen isotope, deuterium, has a higher critical temperature. Hydrides and deuterides of palladium have intensively been studied both experimentally and theoretically, but the origin of the isotope effect has not been established with certainty. The commonly accepted explanation is that the effect is likely to be due to the strong anharmonicity of the optical hydrogen vibrations, which was considered to be responsible for the large deviation of the ratio of the fundamental optical frequencies ωH/ωD = 1.51 from the harmonic value 2 ≈ 1.41. In the present paper, powder samples of MoH1.1(1) and MoD1.07(3) were synthesized under a hydrogen / deuterium pressure of several gigapascals and studied by inelastic neutron scattering (INS) at ambient pressure and T = 10 K. The INS study demonstrated that optical vibrations of H atoms in MoH1.1 and D atoms in MoD1.07 are harmonic and the ratio of fundamental optical frequencies ωH/ωD = 1.44 is close to the harmonic value 2 ≈ 1.41. This shows that anharmonicity is not a necessary condition for the presence of the inverse isotope effect. The MoD1.07 sample was additionally studied by neutron diffraction (ND) at ambient pressure and T = 100 K. In agreement with previous ND results for MoH1.2, the ND study of MoD1.07 showed that deuterium atoms occupied almost all octahedral interstitial sites in its hexagonal close-packed metal lattice and formed a NiAs-type crystal structure with the composition close to MoD. The overstoichiometric composition MoD1.07 of the deuteride is likely to result from a small fraction D/Mo ~ 0.07 of deuterium atoms partially occupying the tetrahedral interstices.