We report NMR studies of the effect of lattice geometry on the orientational ordering of "N, molecules in solid N2-Ar mixtures. The N2 molecules interact through highly frustrated electrostatic quadrupole-quadrupole interactions and samples with 50% N2 that form fcc lattices show evidence for long range order with a very small distribution of order parameters in contrast to the quadrupolar glass phase observed for the hcp lattice at higher concentrations (57% -77%). PACS numbers: 64.70.Kb, 75.50.Lk, 76.60. -k The phenomenon of frustration occurs in a wide variety of apparently disparate systems [1,2] and results from the presence of either strongly competing interactions (magnetic or elastic) or a fundamental geometrical incompatibility between the symmetry of the interactions and that of the underlying lattice structure, or a combination of both features. These systems include the spin glasses [1] (competing magnetic interactions), mixed dipolar crystals [Rb"(NH4)t "H2PO4 [3], Rb"(ND4)~"DqPO4 [4]] (competing ferroelectric and antiferroelectric interactions), mixed molecular crystals (ortho-para H2 and D2 mixtures [5], KCN-KBr [6], Nq-Ar [7 -9], CH4-Kr [10])(frustrated anisotropic interactions), and pyrochlores (Tb2Mo207 [11],CsNiCrFs [12]) with purely geometrically frustrated antiferromagnetic interactions of spins at vertices of corner-connected tetrahedra. All these systems exhibit a number of common features, notably a disordered glasslike low-temperature state with strong local correlations but no long range order and extremely slow polydispersive relaxation processes (" glassy" dynamics) below characteristic temperatures. Systematic studies of prototype systems with strong frustrations are especially important as the insight gained from model systems is expected to have implications for other fields of physics such as neural networks, computing structures, and molecular diffusion on complex macromolecules. With this goal in mind we have studied simple molecular crystals (N2-Ar mixtures) for which the interactions are well understood (electrostatic quadrupolequadrupole), and the differing roles of geometrical frustration and substitutional disorder can be explored by varying the concentration of the noninteracting Ar atoms. There is also a clear distinction between quenched disorder [random occupation of lattice sites by the interacting quadrupole moments (N2)] and the thermal disorder. Furthermore, the local order parameters (molecular alignments) can be measured directly by NMR techniques. The frustration in the mixed molecular systems (N2-Ar, ortho-para H2 mixtures) arises from (i) the topological incompatibility of realizing the minimum possible energy configuration for all pairs of the neighboring quadrupoles on close packed 3D lattices, and (ii) the disorder from the replacement of a quadrupole by a noninteracting (" spherical" ) atom such as Ar in the Nq-Ar system. Despite the topological frustration, a long range periodic orientational order is observed for the cubic lattice that occurs at high N2 co...

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Physics Letters A 256 (1999) 75-80. doi:10.1016/S0375-9601(99)00217-0Received by publisher: 1999-03-19Harvest Date: 2016-01-04 12:22:22DOI: 10.1016/S0375-9601(99)00217-0Page Range: 75-8

An ac capacitance bridge with excellent stability and two parts per billion resolution in the real part of the dielectric constant at temperatures ranging from 4.2 to 80 K and in the audio frequency range (0.2–20 kHz) is reported. The method uses commercially available components as well as few homemade cables and capacitors. Construction details and data on the resolution are presented.

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