The exact mean-field phase behavior of the Semenov-Vasilenko model for rod-coil diblock copolymers is studied by applying self-consistent field techniques. The behavior depends on three quantities: the rod/coil immiscibility N, the coil volume fraction f , and the ratio of the characteristic coil to rod dimensions. When NՇ5, the rods and coils mix producing a nematic phase, and at larger N, they microphase separate forming a lamellar phase. The nonlamellar phases expected at f տ0.7 are not treated here. The lamellar phase is typically a smectic-C structure with monolayers of tilted rods. A thorough understanding of the model is achieved by closely examining segment distributions and various contributions to the free energy. The tilt angle is generally controlled by a competition between rod/coil interfacial tension and stretching of the coils. Lowering f reduces the latter contribution, causing →0 and producing a continuous transition to a smectic-A structure. Beyond that, there is a tendency to form structures with the rods arranged in bilayers, but this is strongly suppressed by a large steric penalty. We suggest that small amounts of solvent can greatly alleviate this penalty, and therefore could significantly affect some aspects of the phase behavior.

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The lattice constants and the atomic position parameter, z, of a high purity, low strain, single‐crystal of arsenic have been determined. Low extinction reflections of filtered Mo Kα and Ag Kα radiation were used for the determination of z. The Bond precision technique with Mo Kα radiation was used for the determination of the unit‐cell dimensions a and c. Average values for a and c in Å, and for z are: 4.2°K z = 0.22764 a = 3.7597 c = 10.4412; 78°K z = 0.22754 a = 3.7595 c = 10.4573; 299±3°K z = 0.22707 a = 3.7598 c = 10.5475. The estimated standard deviation of z is ±0.00004 at 4.2°K, ±0.00002 at 78°K and ±0.00005 at 299°K. The precision of a is estimated to be ±0–0001 Å and that for c to be ±0.0002 Å. The results for As are compared with those reported earlier for Sb and Bi.

The parameters in the crystal structure of α-GeTe were determined by least-squares analysis of intensity data obtained with an automatic x-ray powder diffractometer. The rhombohedral cell containing 4 Ge and 4 Te atoms has a=5.996±0.001 Å, α=88.18°, and the single atom-position parameter x=0.237±0.002. The displacement of one sublattice with respect to the other from the relative position that would correspond to the NaCl structure (i.e., x=0.250) corroborates the predictions, based on band calculations, of Cohen, Falicov, and Golin.