[en] It has been known for quite some time now that proton dynamics plays a key role in the structural ferroelectric (FE)/antiferroelectric (AFE) phase transition in the crystals belonging to the potassium dihydrogen phosphate crystal family. Mixed crystals belonging to this family having the composition M_1−x(NW₄)_xW₂AO₄, where M = K, Rb, Cs, W = H, D, and A = P, As, exhibit proton glass behavior due to frustration between FE and AFE ordering; these proton glasses do not undergo any structural phase change but retain their room temperature structure down to very low temperatures. Single crystal neutron diffraction investigations of four mixed crystals with composition (K_1−x(NH₄)_xH₂PO₄), where x = 0.0, 0.29, 0.67 1.0, were undertaken with the intention to investigate the effect of the local structural deviations on the overall average structure of the crystals and correlate these structural changes to the presence or absence of a structural phase transition in these crystals. Hydrogen bonding is shown to play a key role in the changing nature of the mixed crystals as the composition varies from the potassium rich ferroelectric region to the proton glass region to the ammonium rich antiferroelectric region. (paper)

[en] The native form of serum albumin is the most important soluble protein in the body plasma. In order to investigate the structural changes of Bovine serum albumin (BSA) during its unfolding in the presence of urea, a small-angle neutron scattering (SANS) study was performed. The scattering curves of dilute solutions of BSA with different concentrations of urea in D₂O at pH 7.2 ± 0.2 were measured at room temperature. The scattering profile was fitted to a prolate ellipsoidal shape (a, b, b) of the protein with a = 52.2 A and b = 24.2 A. The change in the dimensions of the protein as it unfolds was found to be anisotropic. The radius of gyration of the compact form of the protein in solution decreased as the urea concentration was increased. (author)

[en] Crystal structures of triglycine selenate (TGSe) and triglycine sulfate (TGS) obtained from single crystal neutron diffraction are compared. The double well single cell local potential experienced by the non-planar amino group of one of the three glycine ions (GI) of these two isostructural crystals is obtained using their crystal structure. It is suggested that the change in the nature of the ferroelectric phase transition as one goes from TGS to TGSe is due to the increase in the zero point energy resulting due to the change in the shape and height of the double well local potential of these crystals. Substitution of a selenate ion (SeO₄^2-) in TGSe by a sulfate ion (SO₄^2-) is considered as a source of an effective chemical pressure that can be utilized to tune the ferroelectric phase boundary in these crystals. The influence of alanine substitution on the ferroelectric phase transition in these crystals is investigated using differential scanning calorimetry.

[en] The two most important molecular movements which bring about the order-disorder ferroelectric phase transition in the hydrogen-bonded ferroelectric triglycine sulfate (TGS) are the swinging of the amino group (-NH₃⁺) of one of its three glycine ions, namely GI, and the tunnelling of hydrogen in the hydrogen bond between its other two glycine ions, GII and GIII (GII-H-GIII). The potential function for bent hydrogen bonds is used along with the structural parameters of the TGS crystal to model the double-well potential (U) seen by the amino group (-NH₃⁺) of GI in TGS. The ferroelectric phase transition in TGS is investigated from the point of view of the double-well instability. Results obtained are in good agreement with those obtained earlier using the Ising-type theoretical model. Correlation between the two crucial molecular movements in TGS, namely swinging of the -NH₃⁺ group of GI and tunnelling of hydrogen in the hydrogen bond GII-H-GIII of TGS, is established

[en] Using the crystal structure, a comprehensive interpretation of the origin of ferroelectricity in the hydrogen bonded triglycine family of crystals is given. Our detailed analysis showed that the instability of nitrogen double well potential plays a driving role in the mechanism of the ferroelectric transitions in these crystals. (author)

[en] One of the most intriguing facts about hydrogen bonds is that bonds formed between the same couple of donor and acceptor atoms can have large variation in the geometry as well as energies; this is mainly because hydrogen bonds are highly sensitive to their environment. We report here a comparative study of neutron structures of mixed crystals of potassium–ammonium dihydrogen phosphate. Basic structural framework of crystals belonging to potassium dihydrogen phosphate family is constructed by a network of zigzag O–H–O hydrogen bonded chains of ${\text{PO}}_2{\text{(OH)}}_2$ anions, with cations occupying the interspaces of the zigzagged anionic chains. These crystals having simple crystal structure primarily determined by the O–H–O hydrogen bonds connecting ${\text{PO}}_2{\text{(OH)}}_2$ anions provide us with a unique opportunity to tune the hydrogen bond geometry by varying the cationic occupancy and study the effect of this change on the average structure of the crystal. The cation–anion interaction is found to have a direct influence on the O–H–O hydrogen bonds of the mixed crystals.

[en] Disodium uridine 5^'-monophosphate heptahydrate (5^'-UMPNa₂), Na₂[C₉H₁₁N₂O₉P].7H₂O, crystallises in space group C222₁ with a=22.985, b=8.911 and c=19.494 A. A neutron beam of λ=1.216 A was used; Z=8 and V=3992.75 A³. Data consisted of 1785 unique reflections. Na ions were connected to the main molecule through water molecules and sugar oxygens. One of the Na ions occupied a special position, and the other at a general position was partially disordered. The uracil base was planar, and had anti conformation about the glycosidic bond. The sugar had C(2^') endo conformation and was gauche-gauche. (orig.)

[en] The crystal structure of ZTS has been determined by neutron diffraction with a final R-value of 0.026. Using the structural parameters, the contributions from the structural groups to the linear optical susceptibility and linear electro-optic coefficients have been evaluated. Results showed a significant contribution from the hydrogen bonds in the structure. (author)

[en] Single layers of Cu and Si and Cu/Si multilayers were deposited on glass substrates kept at room temperature, by r.f. sputtering technique in Argon plasma. The films have been characterised by x-ray reflectometry at Cu Kα (1.54 A) radiation and phase modulated ellipsometry in the wavelength range 300-800 nm. It has been found by the small angle x-ray reflectivity measurement that the sputtering system is able to produce high quality layers with very low roughness (∼ 5 A), which is very much needed for the fabrication of x-ray multilayer mirror systems. The multilayer coating, consisting of 10 bilayers of Cu and Si, have showed Bragg peak up to the 3rd order and about 45% reflectivity at the 1st Bragg peak. (author)

[en] Short communication