Low Bulk Modulus Research Articles

A predicted non-layered phase of In2Se3 by first principles

In2Se3 is an important compound having a complex phase diagram. In this work, we found a non-layered phase of In2Se3 with the help of the particle swarm optimization method. Studies indicate that this phase is dynamically stable. It is an indirect band gap semiconductor with a gap of 2.36 eV. Calculated elastic constants imply that it is mechanically stable with elastic anisotropy. It has a low bulk modulus and Debye temperature despite of its non-layered structure. There are five strong peaks in the simulated infrared absorption spectrum and the corresponding vibrational modes are presented. Born effective charges and dielectric constants are obtained and discussed. The computed birefringence indicates that it might be a potential nonlinear optical crystal.

Removal of Cs+ and Sr2+ ions from simulated radioactive waste solutions using Zeolite-A synthesized from kaolin and their structural stability at high pressures

Zeolite-A was synthesized from kaolin activated at 500 °C at relatively lower temperature than the studies reported in the literature.The metakaolin formed at 500 °C was used to synthesise zeolite-A by treating with 1–5 M NaOH solutions at 80–100 °C for 3–24 h under hydrothermal reaction condition. The zeolite-A was characterized by XRD, FT-IR and SEM. The percentage of crystallinity and crystallite sizes of the zeolite-A were determined from XRD analysis; the crystal system of zeolite-A was found to be cubic with space group (Fm3‾c ). The ion-exchange property of zeolite-A for Cs+ and Sr+2 was explored at ambient temperature (25 °C). The adsorption efficiency of zeolite-A is higher for Sr+2 than that of Cs+; and adsorption capacities for Cs+ and Sr2+ ions were found to be 91.51% and 99.16% respectively. Strontium substituted zeolite has higher crystallinity and lattice strain than that of caesium substituted zeolite-A. Structural stability investigations were carried out on pristine zeolite-A and its Cs+ and Sr2+ substituted analogues by using high-pressure (0.1 MPa-11.4 GPa) X-ray diffractometry technique for the first time. The pristine zeolite-A shows comparatively lower bulk modulus than that of caesium and strontium loaded zeolite. It is also seen that the compressibility is least for the strontium loaded zeolite-A (SrZtA) followed by caesium loaded zeolite-A (CsZtA) and pristine zeolite-A (ZtA) (i.e compressibility of SrZtA < compressibility of CsZtA < compressibility of ZtA).

Experimental equation of state of 11 lanthanide nitrides (NdN to LuN) and pressure induced phase transitions in NdN, SmN, EuN, and GdN

Through an extensive data analysis of powder X-ray diffraction data obtained at pressures up to at least 78 GPa, we report the experimental equations of state for all lanthanide nitrides between NdN and LuN, excluding the radioactive Pm. By fitting the obtained unit cell volumes as a function of pressure with the third order Birch–Murnaghan equation of state, we find that the bulk modulus increases with an increasing lanthanide number from K0 = 146(12) GPa for NdN to 182(7) GPa in EuN. Hereafter, the bulk modulus reaches a plateau for the rest of the series except for TmN which has a lower bulk modulus. We find that the first derivative of the bulk modulus is around 4 for all compounds except TbN, which displays a significantly different compression behavior. In addition, we find a B1 to B10 pressure-induced phase transition in NdN, SmN, EuN, and GdN at increasingly higher pressures. In fact, we observe that the onset pressure of the phase transition increases linearly with Ln atomic number. From TbN and onwards, we do not observe any sign of a B1 to B10 transition indicating that the transition pressure exceeds the studied pressure range. Therefore, we believe that, for the heavier lanthanides, the linear relationship between the onset pressure and the lanthanide number does not hold and even higher pressures are needed to observe the transition. This coherent study of the series of lanthanide nitrides offers a unique opportunity for benchmark studies of computational methods applied to compounds with 4f electrons.

Mechanical and acoustic properties of a hybrid organic–inorganic perovskite, TMCM-CdCl3, with large piezoelectricity

Piezoelectric hybrid organic–inorganic perovskites have attracted significant attention due to their outstanding piezoelectricity comparable to perovskite oxides. However, their elastic properties, which determine the piezoelectric performance, have not yet been explored. Here, the elastic properties of a perovskite ferroelectric TMCM-CdCl3 (TMCM = trimethylchloromethyl ammonium) with a large piezoelectric effect are comprehensively investigated. High-pressure synchrotron powder x-ray diffraction experiments reveal that it exhibits a low bulk modulus of 14.58 GPa, which is an order of magnitude lower than those of perovskite oxides. In addition, Young’s moduli and shear moduli were calculated using density functional theory, and their values are significantly smaller than perovskite piezoceramics. Furthermore, the acoustic properties of TMCM-CdCl3 were calculated. Its longitudinal wave velocity is 3.86 km s−1–4.94 km s−1, and the corresponding specific acoustic impedance is 8.28 MRayl–10.21 MRayl. These maximum values are, respectively, 31.5%, 74.4%, and 8.4%, 73.1% lower than those from BaTiO3 and PbTiO3. Our findings disclose the soft nature and low acoustic impedance of hybrid perovskite piezoelectrics, which are critical to their applications in energy conversion devices.

Pressure-induced decomposition of binary lanthanum intermetallic compounds

We present a comprehensive study on structural and electronic properties of lanthanum intermetallic compounds (${M}_{x}{\mathrm{La}}_{y}$, $M=\text{Be}$, Mg, Al, Ga, In, Tl, Pb, and Bi) under high pressure. By using a swarm intelligence structure search method combined with first-principles calculations, pressure-induced phase transitions of ${M}_{x}{\mathrm{La}}_{y}$ were investigated, with several new structures predicted. A universal yet intriguing phenomenon was found; that is, all of these compounds will decompose into elemental solids at certain pressures, which is against the general intuition that extreme pressure always stabilizes and densifies materials. Mechanical analysis suggests that this anomalous behavior is associated to the elastic moduli and interatomic interaction in ${M}_{x}{\mathrm{La}}_{y}$, and their changes under extreme pressure. A low bulk modulus and larger atomic volume of La result in a smaller volume for the elemental mixture compared to their compound at high pressures, which leads to an energetically favorable $PV$ work and enthalpy for the elemental mixture. Furthermore, the external pressure tends to weaken the La-$M$ electrostatic interaction in compounds as evidenced by the reduced charge transfer between La and $M$, which in turn modifies the electronegativity of La and $M$ and destabilizes the compounds. Our results shed light on the high-pressure behaviors of La-based intermetallic compounds and provide important guidance for understanding other La-like intermetallic compounds at high pressures.

Thermal expansion of Cu/carbon nanotube composite wires and the effect of Cu-spatial distribution

We report for the first time the coefficient of thermal expansion (CTE) of Cu/carbon nanotube (Cu/CNT) macroscopic wire composites and its strong dependence on CNT-Cu mixing. Homogeneous composite wires with 40 vol% nanotubes uniformly mixed in continuous Cu matrix showed CTE ∼4.42 × 10−6 °C-1 (∼26% Cu-CTE and similar to Si-CTE). The value matches with Turner’s model predictions implying that when nanotube-Cu shared contact is high, both low thermal expansion and high bulk modulus of CNTs contribute to offsetting Cu thermal expansion. In contrast, absence of or non-uniform CNT-Cu mixing i.e., reduced shared interface led to higher wire-composite CTEs (∼53−92% Cu-CTE) even with higher CNT vol%.

Robust solid-state interface with a deformable glass interlayer in sulfide-based all-solid-state batteries

We introduced a new elastic interface layer (EIL), (Li2S)0.75(P2S5)0.25 glass, in all-solid-state batteries (ASSBs) to improve the solid-state interfacial structure, and analyzed its structural evolution in electrochemical charge/discharge cycles. This EIL had a lower bulk modulus, thereby providing good powder processability and high mechanical deformability against volume changes of the electrode. Compared to a cell configuring EIL, the reference cell exhibited a considerable increase in the electrolyte resistance; moreover, a new resistance component was identified in the high-frequency region (500–50 kHz). This high-resistance component resulting from interfacial deterioration such as metal fragmentation and micropore formation was successfully suppressed by applying a mechanically deformable EIL. We believe that this study would provide insights into the application of EIL for configuring a stable interface with a metallic electrode in ASSBs.

Identification of Damping Characteristics of EPDM-RUBBER with applications to sandwiched beams and considerations to Engine Mounts for Performance Evaluation

Vibrations are undesirable in many fields owing to its adverse effects therefore as a control procedure, methods have been developed to prevent these vibrations. One of the methods is using dampers. Rubbers are widely used for damping the vibrations of various structures because of its low elastic modulus and high bulk modulus. Behavior of rubber can be classified as Hyper-elastic and Visco-elastic. In FEA rubbers or more precisely elastomers are modeled as Hyper-elastic. Hyper-elastic is a type of constitutive model for rubber like materials and provides a means of specifying their behavior which cannot be exactly specified by linear elastic models. In view of this an attempt is made to identify the damping characteristics of EPDM-RUBBER-rubber, which were identified by using sandwiched structures. Then the engine mount was modeled using this EPDM-RUBBER-rubber as a Hyper-elastic rubber, using the best fit Hyper-elastic material model, and then the performance of selected engine mounts with EPDM-RUBBER as their core rubber was studied for static, harmonic and time varying loads.

Double replication for characterizing cracks in surface-hardened polydimethylsiloxane

Surface-hardened polydimethylsiloxane (PDMS) with random cracks is studied by means of double replication. The PDMS samples are prepared under different curing conditions, resulting in a different Young's modulus of the bulk. To modify the surface, an excimer lamp at 172 nm is used. The samples are stretched uniaxially until the first cracks appear. As an evaluation under strain is hard or rather impossible, the idea is to replicate the cracked sample in a curable material and to characterize the cracks by inspection of the replica. To protect the sample from mechanical loading, this replication is done by molding in OrmoStamp on glass; these replicas are used for optical inspection to determine the crack spacing. As a characterization of the depth and width of the cracks is highly facilitated when cleaving is enabled, a second replication is performed into a thin layer of SU-8 on Si; these second replica are analyzed by secondary electron microscopy of cross sections. They provide a realistic picture of the crack shape. It is found that the curing conditions affect the crack shape; a U-shape occurs with a low bulk modulus, whereas a V-shape occurs with a high bulk modulus. The parameters width, depth, and spacing are largely unaffected by the curing conditions. This work provides a background to understand the behavior of random cracks, which is, e.g., useful to design a system with controlled cracks that remain stable. An important finding is that as soon as cracking occurs at a certain strain, already a number of cracks develop, yet without any further stretching. The cracks behave independently from neighboring cracks. Upon further stretching, new cracks develop, and the crack width and depth remain similar. Therefore, these random cracks are stable, and sample failure does not occur up to a strain of at least 40%.

Ab initio Insight Into the Structure and Properties of Zr–Si System

A first‐principles investigation on the electronic structure, mechanical, and thermal properties of Zr–Si compounds has been investigated to prompt the development of new materials. The influence of Si content on mechanical and thermal properties is clarified. All the Zr–Si compounds are strikingly incompressible under uniaxial stress along x, y, and z axes. β‐ZrSi possesses the strongest resistance to shape change and uniaxial tensions. α‐ZrSi presents better performance of the resistance to volume change. The bulk modulus first increases and then decreases with the increase of the Si concentration. All the compounds exhibit brittle behavior. These zirconium silicides have relatively low bulk and shear modulus, but the shear anisotropy is strongly dependent on diverse directions. Debye temperature increases linearly with the increase of Si content. ZrSi2 shows the best capacity of thermal conductivity. The minimum thermal conductivity in all the crystallographic directions is progressively increased with the increase of Si content except for β‐ZrSi in the [010] direction.

Assessment of corneal substrate biomechanics and its effect on epithelial stem cell maintenance and differentiation

Whilst demonstrated extensively in vitro, the control of cell behaviour via modulation of substrate compliance in live tissues has not been accomplished to date. Here we propose that stem cells can be regulated solely through in situ modulation of tissue biomechanics. By first establishing, via high-resolution Brillouin spectro-microscopy, that the outer edge (limbus) of live human corneas has a substantially lower bulk modulus compared to their centre, we then demonstrate that this difference is associated with limbal epithelial stem cell (LESC) residence and YAP-dependent mechanotransduction. This phenotype-through-biomechanics correlation is further explored in vivo using a rabbit alkali burn model. Specifically, we show that treating the burnt surface of the cornea with collagenase effectively restores the tissue’s mechanical properties and its capacity to support LESCs through mechanisms involving YAP suppression. Overall, these findings have extended implications for understanding stem cell niche biomechanics and its impact on tissue regeneration.

Structure–Property Correlation Studies of Potassium 4,4′-Bis(dinitromethyl)-3,3′-azofurazanate: A Noncentrosymmetric Primary Explosive

The knowledge of mechanical, vibrational, electronic properties, and origin of polarizability of a noncentrosymmetric explosive materials plays a crucial role in understanding the phase transition mechanism and stand-off detection of high energy material (HEM) residues on the surfaces and interfaces. In the present study, we have explored the role of crystal structure and chemical composition in predicting the structural, dynamical, electronic, and optical properties of two newly synthesized noncentrosymmetric green primary explosives, potassium 4,4′-bis(dinitromethyl)-3,3′-azofurazanate (K2BDAF) and potassium 1,1′-dinitramino-5,5′-bistetrazolate (K2DNABT), using density functional theory simulations. The calculated structural and mechanical properties suggest that K2BDAF and K2DNABT are mechanically stable and possess lower bulk modulus values [K2BDAF (18.91 GPa) < K2DNABT (22.4 GPa)] than toxic lead azide Pb(N3)2 (26 GPa). The Born effective charge (BEC) and vibrational, thermodynamic, and phonon dispersion studies reveals that both crystals are dynamically stable and that K2BDAF is found to be more stable, more polarizable, and easily detectable than K2DNABT. The TB-mBJ electronic band structure study reveals that K2BDAF is an indirect band gap (along R → V high symmetry points) material with a gap of 2.37 eV < K2DNABT (3.57 eV, direct). The density of states and charge density plots show that both crystals possess mixed covalent and ionic bonding nature, and these results are consistent with BEC analysis. The nitrogen, oxygen and carbon p states are found to be dominant near the Fermi level in the valence band. From the optical properties study, it is found that the studied explosive materials have the lowest dielectric constant compared with the well-known 4-N,N-dimethylamino-4-N-methylstilbazolium tosylate (DAST) crystal, i.e., K2DNABT (2.68) < K2BDAF (2.91) < DAST (5.2), and show birefringence similar to (K2BDAF static value, 0.32; at 532 nm, 0.58), (K2DNABT static value, 0.25; at 532 nm, 0.27) that of DAST (0.39, 0.55, 0.64) and 4-N,N-dimethylamino-4′-N′-methylstilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) (0.45, 0.63) NLO crystals. For both the studied explosives, the absorption is found to be strong in the [010] direction and follows the relation α(K2BDAF) < α(K2DNABT). The analysis of differences in BEC reveals that from K2DNABT to K2BDAF, the positive contribution of atoms to total birefringence increases as follows: K1 (0.09559), O1 (0.69236), O2 (0.77962), N1 (1.41018), N2 (0.92394), N3 (0.27415), N4 (0.74264), and N5 (0.84502). The negative contribution from C1 is increased by −1.29582. Over all, our results are in agreement with the experimentally reported sensitivity trend, and the reasons behind the sensitivity differences of K2DNABT and K2BDAF are explored.

The Effect of Lanthanum Doping and Oxygen Vacancy on Perovskite, Pyrochlore Oxide and Lanthanide Titanates: A First Principle Study

AbstractThe effects of La-substitution into SrTiO3 (STO) perovskite oxides on their phase structure, formation enthalpy and electrical conductivity have been investigated. La substitution in STO has been reported to show a significant enhancement in electronic conductivity in a wide-band-gap layered perovskite compound STO. Mixture of Lanthanum and Titanium oxide may lead to various phases including La2/3TiO3, La2Ti2O7 and La2TiO5. In this work, more than 50 structural models have been constructed by considering ionic state substituents, distance between substituents and their concentrations. We investigated the formation enthalpy, elastic properties and band gap by density functional theory (DFT) calculations. We have also investigated the effect of reducing environment on La2/3TiO3. The computed bulk modulus (∼2.4 % deviation) and band gap (∼12% deviation) of STO are in good agreement with the literature. Our results indicate that La substitution into STO could significantly reduce the band gap. Reduction in band gap is maximum when the substituents is present at low concentrations. Internal position of La substituents in STO affects the band gap marginally while energy remains almost same. Formation enthalpy of La2/3TiO3 from LaTiO3 is around 2 eV. La2/3TiO3 acts as band insulator (band gap = 2.8 eV). In reducing environment, the band gap of La2/3TiO3 significantly reduces. Sr substitution in La2/3TiO3 lower the band gap and formation enthalpy. La2Ti2O7 and La2TiO5 have higher band gap and lower bulk modulus than STO. Sr substitution is not feasible in La2Ti2O7 and La2TiO5.

Anomalous mechanical behavior of the deltic, squaric and croconic cyclic oxocarbon acids

The structural and mechanical properties of the deltic, squaric and croconic cyclic oxocarbon acids were obtained using theoretical solid-state methods based in Density Functional Theory employing very demanding calculation parameters in order to yield realistic theoretical descriptions of these materials. The computed lattice parameters, bond distances, angles, and x-ray powder diffraction patterns of these materials were in excellent agreement with their experimental counterparts. The crystal structures of these materials were found to be mechanically stable since the calculated stiffness tensors satisfy the Born mechanical stability conditions. Furthermore, the values of the bulk modulus and their pressure derivatives, shear and Young moduli, Poisson ratio, ductility and hardness indices, as well as mechanical anisotropy measures of these materials were reported. A complete review of the literature concerning the negative Poisson ratio and negative linear compressibility phenomena is given together with the theoretical study of the mechanical behavior of cyclic oxocarbon acid materials. The deltic, squaric, and croconic acids in the solid state are highly anisotropic materials characterized by low hardness and relatively low bulk moduli. The three materials display small negative Poisson ratios. The croconic acid displays the phenomenon of negative linear compressibility for applied pressures larger than ∼0.4 GPa directed along the direction of minimum Poisson ratio and undergoes a pressure induced phase transition at applied pressures larger than ∼1.0 GPa.

Gold(i) sulfide: unusual bonding and an unexpected computational challenge in a simple solid.

We report the experimental high-pressure crystal structure and equation of state of gold(i) sulfide (Au2S) determined using diamond-anvil cell synchrotron X-ray diffraction. Our data shows that Au2S has a simple cubic structure with six atoms in the unit cell (four Au in linear, and two S in tetrahedral, coordination), no internal degrees of freedom, and relatively low bulk modulus. Despite its structural simplicity, Au2S displays very unusual chemical bonding. The very similar and relatively high electronegativities of Au and S rule out any significant metallic or ionic character. Using a simple valence bond (Lewis) model, we argue that the Au2S crystal possesses two different types of covalent bonds: dative and shared. These bonds are distributed in such a way that each Au atom engages in one bond of each kind. The multiple arrangements in space of dative and shared bonds are degenerate, and the multiplicity of configurations imparts the system with multireference character, which is highly unusual for an extended solid. The other striking feature of this system is that common computational (DFT) methods fail quite spectacularly to describe it, with 20% and 400% errors in the equilibrium volume and bulk modulus, respectively. We explain this by the poor treatment of static correlation in common density-functional approximations. The fact that the solid is structurally very simple, yet presents unique chemical bonding and is unmodelable using current DFT methods, makes it an interesting case study and a computational challenge.

Mechanical properties of anhydrous oxalic acid and oxalic acid dihydrate.

The mechanical properties of oxalic acid dihydrate and anhydrous oxalic acid (α and β polymorphic forms) were obtained by using rigorous theoretical solid-state methods based on density functional theory using plane waves and pseudopotentials. The calculated crystal structures and X-ray powder diffraction patterns of these materials were found to be in excellent agreement with the experimental information. Since the calculated elasticity matrices fullfilled the Born stability conditions, the corresponding crystal structures were found to be mechanically stable. A large number of relevant mechanical properties including the values of the bulk moduli and their pressure derivatives, shear and Young moduli, Poisson ratios, ductility and hardness indices, and mechanical anisotropy values of these materials were reported. The three forms of oxalic acid are highly anisotropic ductile materials having low hardness and bulk moduli. The three materials are shown to display small negative Poisson ratios (NPR) and to exhibit the phenomenon of negative linear compressibility (NLC) for applied pressures along the direction of the minimum Poisson ratio. In addition, they undergo pressure induced phase transitions for relatively small applied pressures. The analysis of the crystal structures of these materials as a function of pressure demonstrates that the mechanism of NLC of these materials is unrelated to the wine-rack structural mechanism commonly used to rationalize this phenomenon. The three forms of oxalic acid considered in this work are molecular crystals whose structures are characterized by structural elements which are not directly bonded but held together by weak van der Waals forces. The weak bonding between these elements is able to accommodate the structural variations originating from the application of pressure, but the resulting structural deformations appear to be counterintuitive and lead to the anomalous mechanical behavior of these materials.

Two-parameter prestack seismic inversion of porosity and pore-structure parameter of fractured carbonate reservoirs: Part 2 — Applications

Fractures and fracture-related dissolution pores, as well as cavities, molds, and vugs, provide the major conduit and/or storage space for hydrocarbons in the deeply buried carbonate hill of Hexiwu field, Bohai Bay Basin. The fractured reservoir generally has lower porosity but better permeability than moldic/vuggy reservoir, and it consists of the major part of the buried-hill slope and buried-hill internal reservoirs. The conventional method of characterizing carbonate reservoirs, however, often mixes these two types of reservoirs together because they both have low acoustic impedance and low bulk modulus. The rock-physics analysis of two field wells indicates that a pore-structure parameter defined in a rock-physics model, the so-called Sun model, can help to distinguish the fractured reservoir zones together with porosity. Fractured zones usually have porosity of less than 5% and a pore-structure parameter of greater than six, whereas moldic/vuggy reservoirs of higher porosity have a pore-structure parameter of less than six. Field-scale application demonstrates that simultaneous prestack seismic inversion for the porosity and pore-structure parameter enables 3D mapping of fractured reservoir zones in the buried carbonate hills. It also provides an analog of detecting fractures and/or fracture-related pores in deeply buried carbonates in similar geologic settings.

First-principles study of the vibrational characteristics of the heavy element substitution on Cu3SbSe3

Understanding the thermal transport in solids has been an emerging topic since its importance in many fields, such as thermoelectrics. Some typical thermoelectric compounds which has unique vibrational characteristics have been investigated, such as Cu3SbSe3. Experiments have shown that Cu3SbSe3 exhibits abnormally low and nearly temperature-independent lattice thermal conductivity. In this work, we theoretically studied the vibrational characteristics of Ag3SbSe3, Cu3BiSe3 and Cu3SbTe3, which are the substitution of prototype Cu3SbSe3 with heavier elements on each atomic sites. These compounds all exhibit low bulk moduli and group velocities; Ag3SbSe3 and Cu3BiSe3 show larger atomic displacement parameters than Cu3SbSe3 and thus lower Debye temperatures. Both results lead to abnormally low lattice thermal conductivities of these compounds. Furthermore, heavier element substitutions Cu3BiSe3 and Cu3SbTe3 reduce the band gaps, which are expected to be beneficial to higher electrical conductivities than that of Cu3SbSe3. Therefore, due to the lower lattice thermal conductivities and favourable electrical transport properties, Ag3SbSe3 and Cu3BiSe3 are promising thermoelectric candidates.

Structural studies & thermal expansion behavior of samarium Uranate at HP-HT

Sm6UO12 has been synthesized by urea-combustion method and characterized using synchrotron x-ray source. The compound is found to be in single phase crystallizing in rhombohedral lattice with lattice parameters a = 10.153 Å and c = 9.625 Å. High-pressure experiment at ambient temperature reveals the lattice to be stable up to 27.0 GPa. The bulk modulus of the compound is found to be 157.0 GPa. At higher pressures, reflections broaden out as a result of disorder in the system, leading to amorphization at further higher pressures. High pressure and high temperature (HP-HT) x-ray diffraction studies have been carried out up to 6.0 GPa and 673 K. A softening is seen in the material at HP-HT resulting in lower bulk modulus at high temperature. Volume thermal expansion coefficients are found to be 22 × 10−6 K−1 and 17.7 × 10−6 K−1 in the temperature range 298 K–473 K and 298 K-673 K, respectively.

Negative Thermal Expansion, Response to Pressure and Phase Transitions in CaTiF6

Strong volume negative thermal expansion over a wide temperature range typically only occurs in ReO3-type fluorides that retain an ideal cubic structure to very low temperatures, such as ScF3, CaZrF6, CaHfF6, and CaNbF6. CaTiF6 was examined in an effort to expand this small family of materials. However, it undergoes a cubic ( Fm3̅ m) to rhombohedral ( R3̅) transition on cooling to ∼120 K, with a minimum volume coefficient of thermal expansion (CTE) close to -42 ppm K-1 at 180 K and a CTE of about -32 ppm K-1 at room temperature. On compression at ambient temperature, the material remains cubic to ∼0.25 GPa with K0 = 29(1) GPa and K'0 = -50(5). Cubic CaTiF6 is elastically softer and shows more pronounced pressure induced softening, than both CaZrF6 and CaNbF6. In sharp contrast to both CaZrF6 and CaNbF6, CaTiF6 undergoes a first-order pressure induced octahedral tilting transition to a rhombohedral phase ( R3̅) on compression above 0.25 GPa, which is closely related to that seen in ScF3. Just above the transition pressure, this phase is elastically very soft with a bulk modulus of only ∼4 GPa as octahedral tilting associated with a reduction in the Ca-F-Ti angles provides a low energy pathway for volume reduction. This volume reduction mechanism leads to highly anisotropic elastic properties, with the rhombohedral phase displaying both a low bulk modulus and negative linear compressibility parallel to the crystallographic c-axis for pressures below ∼2.5 GPa. At ∼3 GPa, a further phase transition to a poorly ordered phase occurs.