1246 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



crystals and of non-crystalline anisotropic media are classified, and it is 

 pointed out that the possibilities for crystals include smoothly rounded as 

 well as sharp-cornered forms. The condition is formulated for thermo- 

 dynamic stability of a flat crystal face with respect to formation of hill-and- 

 valley structure. A discussion is presented of the limitations on the appli- 

 cability of the results imposed by the dependence of surface free energy on 

 curvature; and it is concluded that these limitations are not likely to be 

 serious for most real substances, though they are serious for certain ideal- 

 ized theoretical models. 



The Crystal Stmctures of NiO-3BaO, NiO-BaO, BaNiOz and Intermediate 

 Phases With Composition Near Ba2Ni20^ ; With a Note on NiO* J. J. Lan- 

 der.^ References. Acta Cryst., v. 4, pp. 148-156, Mar., 1951. 



Abstract — ^The crystal structures of NiO-3BaO, NiO-BaO and BaNiOs 

 have been determined from X-ray diffraction data, and data are given for 

 phases with composition near that represented by Ba2Ni205 . In each of 

 these structures nickel behaves in a novel fashion. A coplanar triangular 

 arrangement of oxygen around nickel is found in NiO-3BaO. In BaNiOs 

 nickel has a valence of four and the structure is a close-packed hexagonal 

 stacking of planar arrangements found in perovskite 111 planes. The com- 

 pound NiO-BaO has a magnetic moment corresponding to two unpaired 

 electrons, whereas the deduced coplanar square arrangement of oxygen 

 around nickel suggests that there should be no unpaired electrons. Com- 

 pounds with composition near Ba2Ni205 contain an amount of oxygen which 

 is a continuous function of temperature and possibly contain mixtures of 

 bi- and tetravalent nickel. 



The problem of NiO having octahedral co-ordination of oxygen is con- 

 sidered. 



New Ferroelectric Tartrates. B. T. Matthias^ and J. K. Hulm. Letter to 

 the editor. Phys. Rev., v. 82, pp. 108-109, April 1, 1951. 



A Negative Impedance Repeater* J. L. Merrill, Jr.^ A.I.E.E., Trans., 

 V. 69, pt. 2, pp. 1461-1466, 1950. 



Inter exchange Tandem Trunking in the Los Angeles Metropolitan Area. 

 W. F. PfeifferI. A.I.E.E., Trans., v. 69, pt. 2, pp. 1071-1079, 1950. 



Abstract — ^Twenty-four years have elapsed since the first large-scale 

 machine switching tandem system was designed and installed for service 

 in Los Angeles. As an intermediate switching center, the tandem office 

 enabled operators to use the dial method of operation for establishing inter- 

 exchange telephone connections over the associated trunking network. 

 During the intervening years, it has facilitated the rapid handling of tele- 



* A reprint of this article may be obtained on request. 

 » Bell Tel. Labs. 



