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Negative thermal expansion was found for ZrW,08 from 0.3 kelvin to its decomposition temperature of about 1050 kelvin. Both neutron and x-ray diffraction data were used to solve and refine the structure of this compound at various temperatures. Cubic symmetry persists for ZrW,08 over its entire stability range. Thus, the negative thermal expansion behavior is isotropic. Essentially the same behavior was found for isostructural HfW,O,. No other materials are known to exhibit such behavior over such a broad temperature range. These materials are finding applications as components in composites in order to reduce the composites' overall thermal expansion to near zero.Negative thermal expansion 1s known in several oxide systems (1, 2). Usually, the contraction is small and anisotropic and occurs only over a small temperature range. W e have studied negative thermal expansion in cubic phases of the type ZrP2-xV,0, (3, 4). Such materials can show negative thermal expansion at high temperatures, but this behav~or does not persist below 100°C for any member of the series. The negative thermal expansion of these materials has been attributed to the transverse vibration of the central 0 in the 0,P-0-PO, or 0,V-0-VO, groups, coupled with frustration In find~ng a structural transition that would allow an ordered bending of P-0-P or V-0-V bond angles (3). By adjustment of x in ZrP2-,V,Oi, materials wlth near-zero thermal expansion can be obtained (3).The synthesis of ZrW20S was descr~bed many years ago (5, 6). This compound was reported to be cubic, but the atomic structure was not determined. Thermal exaansion data above room temperature suggested that negative thermal expansion ceased near room temperature (7). Our results ( Fig. 1) show negative thermal expansion for ZrW20, from 0.3 to 1050 K. Agreement between d l l a t o~n e t r~ data and neutron diffraction data is very good in the region where both tvoes of data were obtained., LThe neutron diffraction data also indicate that ZrW208 is cubic over the entlre temperature range. Both dilatometry data and neutron diffraction data suggest a phase transition near 430 K. It has been reported that H f W 2 0 , is isomorphous with ZrW208(8), but no thermal expansion data weregiven. Our dilatometry data on H f W 2 0 , show negative thermal expansion behavior essentially identical to that of ZrW208, including the phase transition at about 430 K.W e first solved the structure of ZrW20,

Materials that show various responses to multiple external stimuli enable novel device applications. The behavior of systems with strong coupling between magnetic and electronic degrees of freedom provides both challenges for solid-state theory as well as novel phenomena for applications such as colossal magnetoresistance in perovskite manganites. Similarly, a strong coupling between magnetism and dielectric properties in magnetic insulators or semiconductors should lead to devices based on the magnetodielectric effect, where the dielectric properties can be controlled by a magnetic field. Large magnetic-field-induced changes in the resistivity and dielectric properties of La 2 NiMnO 6 are found at temperatures as high as 280 K. This is a much higher temperature than previously observed for such a coupling between the magnetic, electric, and dielectric properties in a ferromagnetic semiconductor. The ferromagnetism of La 2 NiMnO 6 was confirmed through neutron-diffraction studies. La 2 NiMnO 6 is a rare example of a single-material platform with multiple functions, in which the spins, electric charge, and dielectric properties can be tuned by magnetic and/or electric fields.Spintronics (short for spin electronics) is a new technology that combines electronics with magnetics through the manipulation of electron spins. It offers the potential for nonvolatile memories, faster data processing speeds with less power usage, larger storage densities, and additional functionalities, such as quantum computation, which are not possible with conventional semiconductor devices.[1,2] Present spintronic devices, e.g., the giant magnetoresistor (GMR), used in readhead sensors, consist of ferromagnetic metallic alloys wherein spin-dependent scattering and tunneling effects have been successfully applied for commercial use. However, in order to fully achieve the potential of practical spintronic devices, the next generation of spintronic materials should be based on ambient-temperature ferromagnetic semiconductors or heterostructures incorporating ferromagnetic metals with nonmagnetic semiconductors, which enable their easy integration into existing electronic devices. The search for semiconducting materials that exhibit strong ferromagnetic behavior at or above room temperature has been extremely difficult due to conflicting requirements in the crystal structure, chemical bonding, and electronic properties of semiconductors and ferromagnetic materials. [1,3] Generally, ferromagnetic semiconductors and insulators only exhibit magnetic ordering at very low temperatures, e.g., EuS (Curie temperature, T c = 16 K), [4] EuO (T c = 77 K), [5] CdCr 2 Se 4 (T c = 130 K), [6] BiMnO 3 (T c = 100 K), [7] SeCuO 3 (T c = 25 K), [8] and diluted magnetic semiconductors, such as (Ga,Mn)As, [1] which precludes their use in devices. However, one exception is an ordered double perovskite, La 2 Ni 2+ Mn 4+ O 6 , an apparent ferromagnetic semiconductor that has a Curie temperature very close to room temperature (T c~2 80 K); [9±12] this is in the ra...

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Isostructural ZrW2O8 and HfW2O8 show strong negative thermal expansion from 0.3 K up to their decomposition temperatures of approximately 1050 K. This behavior is especially unusual because these compounds are apparently cubic over their entire existence range. Detailed structural studies of ZrW2O8 were conducted using high-resolution neutron powder diffraction data taken at 14 temperatures from 0.3 to 693 K. Below 428 K, ZrW2O8 adopts the acentric space group P213 and has a well-ordered structure containing corner-sharing ZrO6 octahedra and two crystallographically distinct WO4 tetrahedra. Above the phase transition at 428 K, which appears to be second order, the space group becomes centric Pa3̄. The structure is now disordered with one oxygen site 50% occupied, suggesting the possibility of high oxygen mobility. Oxygen motion above 428 K is also suggested by dielectric and ac impedance measurements. The negative thermal expansion of ZrW2O8 and HfW2O8 is related to transverse thermal vibrations of bridging oxygen atoms. These lead to coupled rotations of the essentially rigid polyhedral building blocks of the structure. A semiquantitative model for both the negative thermal expansion and phase transition of these materials is proposed in light of the diffraction results.

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CuCr 1−x Mg x O 2 , a wide band gap semiconductor with the delafossite structure, has been synthesized in bulk and thin-film form. Bulk undoped CuCrO2 is almost black and has moderate conductivity with p-type carriers. Upon doping with 5% Mg, the conductivity increases by a factor of 1000. In films, the best p-type conductivity is 220 S cm−1 in CuCr0.95Mg0.05O2, a factor of 7 higher than previously reported for Cu-based p-type delafossites. Undoped films have a conductivity of order 1 S cm−1. Films are usually polycrystalline on amorphous substrates, but undoped films can be c-axis oriented if deposited at or above 650 °C. Optical and ultraviolet transmission data indicate a direct band gap of 3.1 eV.

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