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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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A long-standing issue regarding the local and long-range structure of V(2)O(5)*nH(2)O xerogel has been successfully addressed. The full three-dimensional structure of the lamellar turbostratic V(2)O(5)*nH(2)O xerogel was determined by the atomic pair distribution function technique. We show that on the atomic scale the slabs of the xerogel can be described well as almost perfect pairs (i.e., bilayers) of single V(2)O(5) layers made of square pyramidal VO(5) units. These slabs are separated by water molecules and stack along the z-axis of a monoclinic unit cell (space group C2/m) with parameters a = 11.722(3) A, b = 3.570(3) A, c = 11.520(3) A, and beta = 88.65 degrees. The stacking sequence shows signatures of turbostratic disorder and a structural coherence limited to 50 A.

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The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co 21 ͞Co 31 charge ordering occur as well as a change in the Co 21 spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering. We report here on structurally related LBaCo 2 O 51x materials, whose structures are derived from perovskites via ordering of the rare earth ͑L͒ and Ba cations into layers along c and removing oxygen exclusively from the L layer [6,7]. This creates an apically connected double layer of corner-sharing CoO 5 pyramids. For x . 0 the extra oxygen ions are incorporated into the L layer of LBaCo 2 O 51x to form disordered octahedra along the c axis. We have synthesized LBaCo 2 O 5.00 and studied the thermal evolution of its structure and properties using synchrotron x-ray [8] (Fig. 1) and the appearance of magnetic superstructure reflections in neutron powder diffraction data. Synchrotron x-ray powder diffraction measurements show that this magnetic transition occurs simultaneously with a tetragonal-to-orthorhombic ͑T -O͒ structural phase transition. Furthermore, as mentioned above, the minority tetragonal phase persists below the T -O transition. The two-phase coexistence was confirmed by ultra-highresolution diffraction measurements using a crystal analyzer. We stress that the observation of two phases in synchrotron x-ray powder diffraction experiments, with an order of magnitude better resolution than available with a standard laboratory x-ray diffractometer, can be traced to minute oxygen-content variations in the order of magnitude around x 0.01 [11]. In the presence of strong lattice coupling these small compositional variations can lead to phase coexistence, which can be observed in high-resolution experiments. This behavior beautifully illustrates how sensitive phase transitions within these systems are to minute variations in the oxygen stoichiometry.At room temperature there is no evidence for any long-range charge ordering. However, upon cooling long-range charge ordering is detected below 200 K, by the appearance of the ͑

The cubic perovskite-related ceramic CaCu 3 Ti 4 O 12 has a very high static dielectric constant 0 տ10 000 at room temperature ͑RT͒, which drops to about 100 below Ӎ100 K. Substituting Cd for Ca reduces the RT value of 0 by over an order of magnitude. The origin of the large 0 is not fully understood, but may be due to an internal barrier layer capacitance ͑IBLC͒ effect. Infrared measurements on the Ca and Cd compounds show that low-frequency modes increase dramatically in strength at low temperature, suggesting a change in the effective charges and increasing electronic localization that may lead to a breakdown of the IBLC effect.High dielectric constant materials find numerous technological applications. In the case of memory devices based on capacitive components, such as static and dynamic random access memories, the static dielectric constant 0 will ultimately decide the level of miniaturization. The dielectric constant of a material is related to the polarizability ␣, in particular, the dipole polarizability ͑an atomic property͒, which arises from structures with a permanent electric dipole which can change orientation in an applied electric field. These two quantities are linked through the ClausiusMossotti relation. In insulators 0 Ͼ0; materials with a dielectric constant greater than that of silicon nitride ( 0 Ͼ7) are classified as ''high dielectric constant'' materials. In general, a value of 0 above 1000 is related to either a ferroelectric which exhibits a dipole moment in the absence of an external electric field, or a relaxor characterized by a ferroelectric response under high electric fields at low temperature, but no macroscopic spontaneous polarization. However, both classes of materials show a peak in 0 as a function of temperature, which is undesirable for many applications. The body-centered cubic perovskite-related material CaCu 3 Ti 4 O 12 shown in Fig.

Powder diffraction patterns of the zeolites natrolite (Na(16)Al(16)Si(24)O(80).16H(2)O), mesolite (Na(5.33)Ca(5.33)Al(16)Si(24)O(80).21.33H(2)O), scolecite (Ca(8)Al(16)Si(24)O(80).24H(2)O), and a gallosilicate analogue of natrolite (K(16)Ga(16)Si(24)O(80).12H(2)O), all crystallizing with a natrolite framework topology, were measured as a function of pressure up to 5.0 GPa with use of a diamond-anvil cell and a 200 microm focused monochromatic synchrotron X-ray beam. Under the hydrostatic conditions mediated by an alcohol and water mixture, all these materials showed an abrupt volume expansion (ca. 2.5% in natrolite) between 0.8 and 1.5 GPa without altering the framework topology. Rietveld refinements using the data collected on natrolite show that the anomalous swelling is due to the selective sorption of water from the pressure-transmission fluid expanding the channels along the a- and b-unit cell axes. This gives rise to a "superhydrated" phase of natrolite with an approximate formula of Na(16)Al(16)Si(24)O(80).32H(2)O, which contains hydrogen-bonded helical water nanotubes along the channels. In mesolite, which at ambient pressure is composed of ordered layers of sodium- and calcium-containing channels in a 1:2 ratio along the b-axis, this anomalous swelling is accompanied by a loss of the superlattice reflections (b(mesolite) = 3b(natrolite)). This suggests a pressure-induced order-disorder transition involving the motions of sodium and calcium cations either through cross-channel diffusion or within the respective channels. The powder diffraction data of scolecite, a monoclinic analogue of natrolite where all sodium cations are substituted by calcium and water molecules, reveal a reversible pressure-induced partial amorphization under hydrostatic conditions. Unlike the 2-dimensional swelling observed in natrolite and mesolite, the volume expansion of the potassium gallosilicate natrolite is 3-dimensional and includes the lengthening of the channel axis. In addition, the expanded phase, stable at high pressure, is retained at ambient conditions after pressure is released. The unprecedented and intriguing high-pressure crystal chemistry of zeolites with the natrolite framework topology is discussed here relating the different types of volume expansion to superhydration.