Enhanced Negative Thermal Expansion Induced by Simultaneous Charge Transfer and Polar–Nonpolar Transitions

High-throughput and high-resolution powder X-ray diffractometer consisting of six sets of 2D CdTe detectors with variable sample-to-detector distance and innovative automation system

The demand for powder X-ray diffraction analysis continues to increase in a variety of scientific fields, as the excellent beam quality of high-brightness synchrotron light sources enables the acquisition of high-quality measurement data with high intensity and angular resolution. Synchrotron powder diffraction has enabled the rapid measurement of many samples and various in situ/operando experiments in nonambient sample environments. To meet the demands for even higher throughput measurements using high-energy X-rays at SPring-8, a high-throughput and high-resolution powder diffraction system has been developed. This system is combined with six sets of two-dimensional (2D) CdTe detectors for high-energy X-rays, and various automation systems, including a system for automatic switching among large sample environmental equipment, have been developed in the third experimental hutch of the insertion device beamline BL13XU at SPring-8. In this diffractometer system, high-brilliance and high-energy X-rays ranging from 16 to 72 keV are available. The powder diffraction data measured under ambient and various nonambient conditions can be analysed using Rietveld refinement and the pair distribution function. Using the 2D CdTe detectors with variable sample-to-detector distance, three types of scan modes have been established: standard, single-step and high-resolution. A major feature is the ability to measure a whole powder pattern with millisecond resolution. Equally important, this system can measure powder diffraction data with high Q exceeding 30 Å-1 within several tens of seconds. This capability is expected to contribute significantly to new research avenues using machine learning and artificial intelligence by utilizing the large amount of data obtained from high-throughput measurements.

Negative Thermal Expansion in Fluoroapatite Pb5(VO4)3F Enhanced by the Steric Effect of Pb2

Negative thermal expansion (NTE) is an unusual thermophysical phenomenon and has gained attention as a way of controlling thermal expansion. Here, we report a substantial NTE in fluoroapatite Pb₅(VO₄)₃F in a limited temperature range. The dilatometric study revealed volume shrinkage below 150 K, giving a linear thermal expansion coefficient of α_L = -44 ppm/K in the temperature range from 140 to 120 K upon heating. The NTE behavior is associated with a structural transition from the hexagonal (P6₃/m) phase to the monoclinic (P2₁/b) phase. Such a structural transition has been found in other apatite-type compounds, but the magnitude of the volume change in Pb₅(VO₄)₃F is remarkable. Our structural analysis revealed that the structural transition is classified as an antiferroelectric-to-paraelectric transition and the volume change during the transition is enhanced by the steric effect of 6s² lone-pair electrons of Pb²⁺.

Chemical Diversity for Tailoring Negative Thermal Expansion

Negative thermal expansion (NTE), referring to the lattice contraction upon heating, has been an attractive topic of solid-state chemistry and functional materials. The response of a lattice to the temperature field is deeply rooted in its structural features and is inseparable from the physical properties. For the past 30 years, great efforts have been made to search for NTE compounds and control NTE performance. The demands of different applications give rise to the prominent development of new NTE systems covering multifarious chemical substances and many preparation routes. Even so, the intelligent design of NTE structures and efficient tailoring for lattice thermal expansion are still challenging. However, the diverse chemical routes to synthesize target compounds with featured structures provide a large number of strategies to achieve the desirable NTE behaviors with related properties. The chemical diversity is reflected in the wide regulating scale, flexible ways of introduction, and abundant structure-function insights. It inspires the rapid growth of new functional NTE compounds and understanding of the physical origins. In this review, we provide a systematic overview of the recent progress of chemical diversity in the tailoring of NTE. The efficient control of lattice and deep structural deciphering are carefully discussed. This comprehensive summary and perspective for chemical diversity are helpful to promote the creation of functional zero-thermal-expansion (ZTE) compounds and the practical utilization of NTE.

Realization of Negative Thermal Expansion in Lead-Free Bi0.5K0.5VO3 by the Suppression of Tetragonality

Bi_1/2K_1/2VO₃ is a lead-free PbTiO₃-type compound with a tetragonality (c/a = 1.054) comparable to that of typical ferroelectric PbTiO₃ (c/a = 1.064) with negative thermal expansion (NTE) during the tetragonal-to-cubic phase transition; therefore, Bi_1/2K_1/2VO₃ is a potential lead-free NTE material if its metastable perovskite structure can be maintained at high temperatures. In the present experiment, electron doping in Bi_1/2K_1/2VO₃ was conducted through substituting K⁺ with La³⁺ to suppress the tetragonality and achieve NTE. La substitution successfully suppressed the tetragonality of Bi_1/2K_1/2VO₃ and also improved its thermal stability. Moreover, both composition- and temperature-induced tetragonal-to-cubic phase transitions occurred. In particular, a large volume shrinkage with a large negative thermal coefficient of expansion (CTE) was obtained for Bi_0.5K_0.46La_0.04VO₃ during the tetragonal-to-cubic phase transition (ΔV = -0.66%). Hence, this study extends the NTE family and also sheds light on the exploration of lead-free piezoelectric materials with controllable thermal expansion.

Anomalous Thermal Expansion in Ta2WO8 Oxide Semiconductor over a Wide Temperature Range

Expansion of material is one of the major impediments in the high precision instrument and engineering field. Low/zero thermal expansion compounds have drawn great attention because of their important scientific significance and enormous application value. However, the realization of low thermal expansion over a wide temperature range is still scarce. In this study, a low thermal expansion over a wide temperature range has been observed in the Ta₂WO₈ oxide semiconductor. It is a balance effect of the negative thermal expansion of the a axis and the positive thermal expansion of the b axis and the c axis to achieve low thermal expansion behavior. The results of the means of variable temperature X-ray diffraction and variable pressure Raman spectroscopy analysis indicated that the transverse vibration of bridging oxygen atoms is the driving force, which is corresponding to the low-frequency lattice modes with a negative Grüneisen parameter. The present study provides one wide band gap semiconductor Ta₂WO₈ with anomalous thermal expansion behavior.

Intermetallic Charge Transfer in V-Substituted PbCrO3

PbCrO₃ features an unusual charge distribution Pb_0.5²⁺Pb_0.5⁴⁺Cr³⁺O₃ with Pb charge disproportionation at ambient pressure. A charge transfer between Pb and Cr is induced by the application of pressure resulting in Pb²⁺Cr⁴⁺O₃ charge distribution and a large volume collapse. Here, structural and charge distribution changes in PbCr_1-xV_xO₃ are investigated. Despite a cubic crystal structure in 0 ≤ x ≤ 0.60, discontinuous reduction in the unit cell volume was observed between x = 0.35 and 0.40. Hard X-ray photoemission spectroscopy confirmed the change in Pb charge state from the coexisting Pb²⁺ and Pb⁴⁺ at x = 0.35 to single Pb²⁺ at x = 0.40. This indicates that V substitution stabilizes the high pressure cubic Pb²⁺Cr⁴⁺O₃-type phase. With further increase in the V substitution, the PbVO₃-type polar tetragonal phase appeared at x = 0.80.

Observation of novel charge ordering and spin reorientation in perovskite oxide PbFeO3

PbMO₃ (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO₃ remain unclear. Herein, we reveal that PbFeO₃ crystallizes into an unusual 2a_p × 6a_p × 2a_p orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb²⁺_0.5Pb⁴⁺_0.5FeO₃ lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb²⁺ and Pb⁴⁺) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe³⁺ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

Stabilized Charge, Spin, and Orbital Ordering by the 6s2 Lone Pair in Bi0.5Pb0.5MnO3

Bi and Pb ions with charge degree of freedom depending on 6s² and 6s⁰ electronic configurations were combined with the Mn ion in a perovskite oxide. Comprehensive theoretical and experimental investigations revealed the Bi³⁺_0.5Pb²⁺_0.5Mn³⁺_0.5Mn⁴⁺_0.5O₃ charge ordered state with CE-type spin and dz² orbital orderings as observed in La_0.5Ca_0.5MnO₃, Nd_0.5Sr_0.5MnO₃, and Bi_0.5Sr_0.5MnO₃. The charge and orbital orderings were preserved above 500 K owing to the stereochemical activity of Bi³⁺ and Pb²⁺ ions which stabilized the structural distortion.