Seismic velocities of CaSiO3 perovskite can explain LLSVPs in Earth's lower mantle.

Seismology records the presence of various heterogeneities throughout the lower mantle1,2, but the origins of these signals-whether thermal or chemical-remain uncertain, and therefore much of the information that they hold about the nature of the deep Earth is obscured. Accurate interpretation of observed seismic velocities requires knowledge of the seismic properties of all of Earth's possible mineral components. Calcium silicate (CaSiO3) perovskite is believed to be the third most abundant mineral throughout the lower mantle. Here we simultaneously measure the crystal structure and the shear-wave and compressional-wave velocities of samples of CaSiO3 perovskite, and provide direct constraints on the adiabatic bulk and shear moduli of this material. We observe that incorporation of titanium into CaSiO3 perovskite stabilizes the tetragonal structure at higher temperatures, and that the material's shear modulus is substantially lower than is predicted by computations3-5 or thermodynamic datasets6. When combined with literature data and extrapolated, our results suggest that subducted oceanic crust will be visible as low-seismic-velocity anomalies throughout the lower mantle. In particular, we show that large low-shear-velocity provinces (LLSVPs) are consistent with moderate enrichment of recycled oceanic crust, and mid-mantle discontinuities can be explained by a tetragonal-cubic phase transition in Ti-bearing CaSiO3 perovskite.

High-temperature ab initio calculations on FeSi and NiSi at conditions relevant to small planetary cores.

The Fe-Ni-Si system is potentially a very important component of terrestrial planetary cores. However, at present, even the behaviour of the FeSi and NiSi end members is poorly understood, especially at low to moderate pressures-the data for FeSi are contradictory and NiSi has been little studied. For FeSi, there is general agreement that there is a phase transition from the ε-FeSi to the CsCl structure with increasing pressure, but, in experiments, there is disagreement as to the position and slope of the phase boundary and the range of coexistence of the two phases. In this paper we have used ab initio lattice dynamics calculations to determine the phase boundary between the ε-FeSi and CsCl structures as a function of pressure and temperature in both FeSi and NiSi. For FeSi, we find that the transition pressure at zero Kelvin is ~11 GPa and that the boundary between the ε-FeSi and CsCl phases varies little with temperature, having a slight negative Clapeyron slope, going from ~11 GPa at 300 K to ~3 GPa at 2000 K. For NiSi, there is much greater variation of the transition pressure with temperature, with a much shallower negative Clapeyron slope, going from ~156 GPa at 300 K to ~94 GPa at 2000 K.

Neutron powder diffraction studies of sulfuric acid hydrates. II. The structure, thermal expansion, incompressibility, and polymorphism of sulfuric acid tetrahydrate (D2SO4.4D2O).

We report results of the first neutron powder diffraction study of sulfuric acid tetrahydrate (SAT); D(2)SO(4)4D(2)O is tetragonal, space group P42(1)c, with two formula units per unit cell. At 1.7 K the unit-cell dimensions are a=b=7.475 12(6) A, c=6.324 66(5) A and V=353.405(5) A(3). At 225 K the unit-cell dimensions are a=b=7.4833(1) A, c=6.4103(1) A, and V=358.98(1) A(3). The deuteron positions refined from the neutron data are in excellent agreement with the single crystal x-ray analysis of Kjallman and Olovsson [Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. B28, 1692 (1972)]; the structure consists of SO(4) (2-) and D(5)O(2) (+) ions hydrogen bonded to form a three dimensional network. Although no structural change is observed between 2 K and the melting point at approximately 232 K, the thermal expansion and incompressibility of the crystal are highly anisotropic. The bulk modulus of SAT at 200 K is 9.2(2) GPa, ( partial differentialK partial differentialP)(T)=7.9(8), and -( partial differentialK partial differentialT)(P)=10.6(5) MPa K(-1), values which are very similar to D(2)O ice Ih. A new polymorph of SAT has been discovered above 235 K at 5.5 kbars. The structure of this phase could not be determined, but we have indexed the diffraction pattern with a monoclinic unit cell of likely space-group P2(1)a (Z=2). SAT-II has a lower density than SAT-I under the same PT conditions; the refined unit-cell parameters at 235 K, 5.435 kbars are a=6.1902(3) A, b=11.1234(5) A, c=5.6446(3) A, beta=110.287(4) degrees , and V=364.56(2) A(3). This phase has been quenched to low pressures and temperatures, and we have obtained estimates of the thermal expansivity and incompressibility which reveal SAT-II to be significantly stiffer and more isotropic than SAT-I.

Optical birefringence study of the ferroelectric phase transition in lithium niobate tantalate mixed crystals: LiNb(1-x)Ta(x)O(3).

The optical birefringence of a complete solid-solution series of lithium niobate-tantalate crystals has been measured as a function of temperature. It is found that, irrespective of composition, the high-temperature paraelectric phase has a birefringence close to +0.063, suggesting that this value arises purely from the oxygen octahedra in the crystal structure. It is also observed that a small addition of lithium niobate to the tantalate produces a crystal that has zero birefringence at room temperature.

Neutron powder diffraction studies of sulfuric acid hydrates. I. The structure of sulfuric acid hemitriskaidekahydrate D2SO4.6(1/2)D2O.

We report the first neutron diffraction data from D2SO4.6(1/2)D2O. The crystal is monoclinic, space group Cm, with four formula units per unit cell. At 4.2 K the unit cell dimensions are a = 6.253 26(4) A, b = 26.813 62(10) A, c = 5.908 45(2) A, and beta = 112.1939(3) degrees [V = 917.286(6) A3 and rho(deuterated) = 1664.14(2) kg m(-3)]. The deuteron positions refined from the neutron data are in agreement with those established by single crystal x-ray analysis [D. Mootz and A. Merschenz-Quack, Z. Naturforsch. B 42, 1231 (1987)], but not with those found from the ab initio simulation of Hirsch and Ojamae [Acta Crystallogr, Sect. B: Struct. Sci. 60, 179 (2004)]. The crystal consists of SO4(2-), D3O+ ions, and D2O molecules hydrogen bonded to form a layered structure in which sheets of "icelike" D3O+ and D2O are separated by layers of opposing SO4(2-) tetrahedra.

No evidence for large-scale proton ordering in Antarctic ice from powder neutron diffraction.

We have examined a sample of 3000 year old Antarctic ice, collected at the Kohnen Station, by time-of-flight powder neutron diffraction to test the hypothesis of Fukazawa et al. [e.g., Ann. Glaciol. 31, 247 (2000)] that such ice may be partially proton ordered. Great care was taken to keep our sample below the proposed ordering temperature (237 K) at all times, but we did not observe any evidence of proton ordering.

Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core.

The nature of the stable phase of iron in the Earth's solid inner core is still highly controversial. Laboratory experiments suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations have indicated the possible presence of complex, inner-core layering. Theoretical studies currently suggest that the hexagonal close packed (h.c.p.) phase of iron is stable at core pressures and that the body centred cubic (b.c.c.) phase of iron becomes elastically unstable at high pressure. In other h.c.p. metals, however, a high-pressure b.c.c. form has been found to become stabilized at high temperature. We report here a quantum mechanical study of b.c.c.-iron able to model its behaviour at core temperatures as well as pressures, using ab initio molecular dynamics free-energy calculations. We find that b.c.c.-iron indeed becomes entropically stabilized at core temperatures, but in its pure state h.c.p.-iron still remains thermodynamically more favourable. The inner core, however, is not pure iron, and our calculations indicate that the b.c.c. phase will be stabilized with respect to the h.c.p. phase by sulphur or silicon impurities in the core. Consequently, a b.c.c.-structured alloy may be a strong candidate for explaining the observed seismic complexity of the inner core.

Differential IL-2 receptor expression in renal allograft recipients treated with an anti-IL-2-receptor antibody.

Patients were entered into a randomized trial of prophylaxis for renal allograft rejection by the administration of an anti-human IL-2 receptor antibody, anti-Tac, during the first ten days posttransplant. Interleukin-2 receptor (IL-2 R) expression was measured using two anti-IL-2 R monoclonal antibodies (moAbs), anti-Tac and 1HT4-4H3. These two antibodies recognize closely spaced epitopes on the 55 kD chain of the IL-2 R. IL-2 R expression was examined on peripheral blood small lymphocytes in three groups of patients who received: (A) cyclosporine CsA and prednisone for baseline immunosuppression (n = 9); (B) anti-Tac with CsA and prednisone as baseline immunosuppression (n = 12); and (C) anti-Tac with azathioprine and prednisone as baseline immunosuppression (n = 5). We found that large numbers of T cells express IL-2 receptors despite the presence of anti-Tac (average of IL-2 R-positive cells at day of peak IL-2 R expression 56.0 +/- 20.8% in group A, 65.2 +/- 26.6% in group B, 21.0 +/- 7.4% in group C). IL-2 R expression did not correlate with clinical activity, and the presence or accessibility of epitopes on the same 55 kD chain varied dramatically from patient to patient.

Decrease in phenotypically defined T helper inducer cells (T4+4B4+) and increase in T suppressor effector cells (T8+2H4+) in stable renal allograft recipients.

Two monoclonal antibodies, anti-2H4 and anti-4B4, reciprocally divide the T4+ (CD4+) and T8+ (CD8+) lymphocytes into T4+2H4+, T4+4B4+, T8+2H4+ and T8+4B4+ subsets. The T4+2H4+, T4+4B4+ and T8+2H4+ subsets possess suppressor-inducer, helper-inducer, and suppressor-effector function, respectively, as previously defined in a system of B cell immunoglobulin production. Using monoclonal antibodies, including anti-2H4 and anti-4B4, and flow cytometry, we monitored lymphocyte subpopulations in 66 renal allograft recipients. We found that patients with stable allograft function have a decrease in the percentage of total T4+ lymphocytes from 41.9 +/- 9.5% pretransplant (pre-Tx) to 36.3 +/- 13.9% posttransplant (post-Tx) (P less than 0.05). This decrease was seen mainly in the T4+4B4+ or helper-inducer subset from 20.8 +/- 4.7% (pre-Tx) to 16.0 +/- 6.3% (post-Tx) (P less than 0.005). Patients with stable function were also noted to have an increase in the percentage of total T8+ lymphocytes from 21.3 +/- 10.7% (pre-Tx) to 30.9 +/- 15.4% (post-Tx) (P less than 0.02). Examination of T8 subsets revealed that a statistically significant increase was seen in the T8+2H4+ or suppressor effector subset from 15.5 +/- 9.2% (pre-Tx) to 21.5 +/- 10.2% (post-Tx) (P less than 0.01). Additionally, serial studies on 14 patients revealed an increase in the %T4+2H4+ suppressor-inducer subset from 9.31 +/- 3.64% (pre-Tx) to 15.71 +/- 6.41% (post-Tx) (P less than 0.0025). Since the role of these subsets has not been established in alloimmunity, in vitro allogeneic studies of 2H4-enriched (2H4+) and 2H4-depleted (2H4-) lymphocytes from normal peripheral blood were performed. In the mixed lymphocyte reaction, 2H4+ cells proliferated less than 2H4- cells (cpm ratio 2H4+/2H4-: 0.63-0.84), but 2H4+ cells generated twice as much suppressor activity as 2H4- cells (ratio % suppression 2H4+/2H4-: 1.9-2.3). These results suggest that 2H4+ cells play a role in the suppressor limb of the alloimmune response and that the increase in cells of this phenotype in our transplant population may be responsible for the maintenance of stable allograft function.