Neutron powder diffraction has been used to characterize a sample of C-substituted MgB 2 synthesized from Mg and B 4 C (with isotopically enriched 11 B). The sample is multiphase, with the major phase [73.4(1) wt.%] being Mg(B 1-x C x ) 2 with x=0.10(2). Minor phases include MgB 2 C 2 , Mg, and MgO. The major Mg(B 1-x C x ) 2 phase displays diffraction peak widths as sharp as for pure MgB 2 , indicating good C homogeneity. There is no evidence for ordering of the substituted C atoms or distortion of the host structure other than contraction of the a axis and slight expansion of the c axis. The observed changes in lattice parameters vs. C concentration provide a means for estimating the C concentration in other Mg(B 1-x C x ) 2 samples. The reduction in T c resulting from 10% C substitution is much larger than previously reported, suggesting that previous reports of the C concentration in Mg(B 1-x C x ) 2 are overestimated. The Mg site occupancy is determined to be 0.990(4) which is consistent with full Mg occupancy. Given these results, the stoichiometry Mg(B 0.9 C 0.1 ) 2 should be used by future attempts (band structural or otherwise) to explain (i) the dramatic suppression of T c (T c ≈ 22 K) and (ii) the persistence of the two-superconducting-gap feature in the specific heat data. PACS: 74.70, 74.62.B, 61.12, 61.66

We report Raman spectra and synchrotron x-ray diffraction measurements of lattice parameters of polycrystalline MgB 2 under hydrostatic pressure conditions up to 15 GPa. An anomalously broadened Raman band at 620 cm Ϫ1 is observed that exhibits a large linear pressure shift of its frequency. The large mode damping and Grüneisen parameter indicate the vibration is highly anharmonic, broadly consistent with theoretical predictions for the E 2g in-plane boron stretching mode. The results obtained may provide additional constraints on the electron-phonon coupling in the system.

Detailed measurements of M ͑2 K, H, ͒ of HoNi 2 B 2 C, where is the angle that the applied field H makes with the ͓110͔ axis while remaining perpendicular to the crystallographic c axis, reveal three metamagnetic transitions with angular dependences H c1 ϭ͑4.1Ϯ0.1 kG͒/cos͑͒, H c2 ϭ8.4Ϯ0.2 kG/cos͑͒, and H c3 ϭ͑6.6Ϯ0.2 kG͒/sin͑͒, where ϭϪ45 is the angle from the ͓100͔ axis. The high-field saturated moment, M sat Ϸ10 B cos is consistent with the local moments being confined to the ͓110͔ direction. The locally saturated moments for fields between H ci ͑iϭ1,2,3͒ also manifest angular dependences that are consistent with combinations of local moments along ͓110͔ axes. Analysis of these data lead us to infer that the net distribution of moments is ͑↑↓↑↓↑↓͒ for HϽH c1 , ͑↑↑↓↑↑↓͒ for H c1 ϽHϽH c2 , ͑↑↑→↑↑→͒ for H c2 ϽHϽH c3 , and ͑↑↑↑↑↑↑͒ for HϾH c3. ͓S0163-1829͑97͒02902-0͔