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Ice-XV

Ice-XV
Ice-fifteen (Ice XV)

V β-ice XV

 

Ice XV is slightly expanded hydrogen-bonding ordered form of the ice VI phase. Its existence was surmised based on the presence of the ordered forms of other ices at low temperatures. Its structure, using the fully deuterated form (D2O) was first described in 2009 [1582]. It was prepared by using 10 mM DCl (deuterated hydrochloric acid, about one molecule to every 5000 molecules of water) as a dopant to help the ordering rearrangement of the hydrogen bonding at low temperatures (<130 K). Hydrogen ordering is faster in H2O (using HCl) ice VI than in D2O ice VI with the process possibly transitioning through ferroelectric intermediates [2371].

 

Ice XV is antiferroelectric with all the water molecule dipoles canceling out as each of the two separate interpenetrating networks possess oppositely aligned hydrogen bonding. As with ice VIII, other ordered but ferroelectric hydrogen bonding is possible but such crystals have not yet been found.

 

The pseudo-orthorhombic crystal (shown opposite), is in P bar1 space group, has cell dimensions a = 6.2323 Å, b = 6.2438 Å, c = 5.7903 Å (90.06º, 89.99º, 89.92º; D2O, at ~0.9 GPa and 80 K) [1582]. During the ordering of the hydrogen bonding of ice VI, the a and b lattice constants contract whereas there is a pronounced expansion in the c lattice constant [2687].

 

The three different hydrogen-bonding ordered structures, from [2687]

the three different hydrogen-bonding ordered structures from [2687]
The two networks with ordered hydrogen bonding can have one of three ordered hydrogen bonding arrangements (A,B,C, see left) giving six different ordered structurers, AA, BB, CC, AB, AC, BC. With rotations of individual networks, there are 45 different crystalline structures possible [2687]. However, it appears that the found structure contains the highly polar C-type networks and is antiferroelectric.

 

Ice-fifteen has triple points with ice-six and ice-two (estimated at 130 K, 0.8 GPa), and ice-six and ice-eight (estimated at 130 K, 1.5 GPa) [1582].

Ice β-XV, possible ice XVIII

It appears (in 2017 [3023]) that another ordered form of ice VI exists that is stable at lower temperatures (<103 K) and different in structure to the form described above. Thus the earlier form of ice-fifteen (still called ice XV ) is stable between 103 K and 129 K and the new form is called ice β-XV and may be called ice XVIII in the future. At present, it is not known which structure ice β-XV has out of the 45 different crystalline structures possible .

 

Interactive Jmol structures of ice VI and ice XV are given.

 

 

 

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This page was established in 2009 and last updated by Martin Chaplin on 8 November, 2017


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