A neutron diffraction study of the Nowotny phase Mo4.88i3C0.6 combined with an X-ray diffraction analysis has been used to determine the C atom positions and to refine the other structure parameters. The compound crystallizes with a hexagonal unit cell with a=7.286+ 0.001, c= 5.046+ 0.002 /~, in space group P63/mcm (D3h), six Mo atoms in position 6(g0 with x = 0.240, only 3.6 Mo atoms in 4(d), six Si atoms in position 6(g~) with x=0.60 and 1"2 C atoms in 2(b). The carbon atoms occupy the octahedral voids formed by the Mo atoms in position 6(g0.It is shown that these carbon-centered transition metal octahedra are an important structural element not only of Nowotny phases but also of a whole group of complex transition metal carbides. From a knowledge of the tendency with which elements like to form T6C octahedra, it is possible to predict the arrangement of the different atoms on the different structure sites of the complex ternary and quaternary carbides. ERWIN PARTHI~, WOLFGANG JEITSCHKO AND VARADACHARI SADAGOPAN 1035
1. The comparative response of naked neck (Nana) broiler chickens and their normally feathered (nana) full sibs to varied dietary energy levels was assessed under natural ambient conditions in a subtropical climate in two separate trials, one in winter (19.0 to 26.0 degrees C) and the other in summer (28.1 to 37.4 degrees C). 2. The chickens were fed on diets with 10.9, 11.5 or 12.1 MJ ME/kg and with a constant protein content of 200 g/kg from 1 to 42 d of age. 3. Both genotypes showed similar body weight, food intake and food conversion efficiency at 42 d of age. Body weight and food conversion efficiency improved on the diet with the highest energy content. 4. Apparent retention (AR) of crude protein (CP) was higher in nana chickens than in their Nana sibs in the winter trial, while no differences were observed for AR of dry matter (DM), crude fibre (CF), ether extract (EE) or calcium. In the summer trial, no difference in nutrient retention was seen between the genotypes. Increasing dietary energy had a positive influence on AR of DM, CP and EE. 5. The relative weights of feathers and skin were lower in Nana chickens than in their normal counterparts. Ready-to-cook yields were greater at the high dietary energy content, while abdominal fat increased with increasing energy content. Nana chickens had more skin protein and less fat in breast muscle and skin than their normal sibs in the summer trial. 6. It was concluded that the Nana broiler chickens showed similar performance and did not differ in dietary energy requirements in comparison to their nana sibs under natural ambient conditions in a subtropical climate.
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