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Hybridization of heavier p block elements
Hybridization of s and p orbitals to form effective sp hybrid orbitals requires that they have comparable radial extent. While 2p orbitals are on average less than 10% larger than 2s, in part attributable to the lack of a radial node in 2p orbitals, 3p orbitals which have one radial node, exceed the 3s orbitals by 20-33%. The difference in extent of s and p orbitals increases further down a group. The hybridization in of atoms in chemical bonds can be analyzed by considering localized molecular orbitals, for example using natural localized molecular orbitals in a natural bond orbital (NBO) scheme. In methane, CH4, the calculated p/s ratio is approximately 3 consistent with "ideal" sp3 hybridization, whereas for silane, SiH4, the p/s ratio is closer to 2. A similar trend is seen for the other 2p elements. Substitution of fluorine for hydrogen further decreases the p/s ratio. The 2p elements exhibit near ideal hybridization with orthogonal hybrid orbitals. For heavier p block elements this assumption of orthogonality cannot be justified. These deviations from the ideal hybridization were termed hybridization defects by Kutzelnigg.

TY - JOUR T1 - Orthogonal and non-orthogonal hybrids JO - Journal of Molecular Structure: THEOCHEM VL - 169 IS - 0 SP - 403 EP - 419 PY - 1988/8// T2 - AU - Kutzelnigg, W. SN  - 0166-1280 DO - http://dx.doi.org/10.1016/0166-1280(88)80273-2 UR - http://www.sciencedirect.com/science/article/pii/0166128088802732

Kutzelnigg in 1986 highlighted carbon as a unique case which adheres most closely to the conventional sp hybridization model.

Lede
Phosphate glasses have been used in lasers (for example Nd doped glass), alkali metal phosphate glasses are sequestration agents for hard water and dispersants for clay processing as well as pigment manufacture. Biocompatible phosphate glases are used in medical applications. Iron phosphate glasses have been investigated for use as hosts for nuclear waste. The phosphate glasses were first investigated by Schott and coworkers  in the early part of last century. 20th century by Schott and coworkers. While the glasses have good optical properties such as high transparency to ultraviolet light they were considered to sensitive to moisture to be generally useful. P2O5 was identified by Zachariesen as one of the prototypical network formers in the 1930's.(Take from Brow, Neel and Salih and Neel Pickup reviews) Glasses formed from multiple network formers have been produced, for example aluminophosphate, silicophosphate as well as molybdophosphate and tungstophosphate glasses formed with molybdenum and tungsten oxide

Structure
Phosphate glasses are based on phosphorus pentoxide, P2O5 as the "network former". Tetrahedral {PO4} units are lnked together by bridging oxygen units. This linking of tetrahedral units is shared with the silicate glasses, which are based on silica as the network former. A structural difference between silicates and phosphates is that while silicates can share all four corners in phosphates a maximum of three corners can be shared. . Crystalline compounds can be prepared as well as amorphous glasses. Phosphate compounds can be classified by their P/O ratio as follows:

Binary, Ternary, Quaternary glasses
Simple binary glasess derived from from P2O5 and a metal single metal oxide, e.g. Na2O, CaO. Ternary has two defferent metal oxides and quaternary 3. The addition of other oxides allows modificaion of the properties of the glasses.

Preparation of glasses
Melt quenching- could use P2O5 but too volatile and readily hydrolysed- use a phosphate compound e.g. metaphosphate and a metal carbonate rather than oxide- metaphosphate and carbonate decompose releasing H2O and CO2 respectively which is useful as it aids in homogenizing the melt. .

Sol-gel methods - allow for unusual oxides such as TiO2 to be incorporated. Alkoxides and ???? form sol- wait for gel- calcine to dehydrate.

Mechanism of glass formation
Add ing oxide to P2O5 causes depolymerization. Van wazer suggested that 2Q2 <-> Q3 +Q1 and 2Q1<-> Q2 + Q0m

Invert glasses
These are called because there have a high P/O ratio - contain small anionic units and properties depend on nature of the cations present.

Polyphosphate glasses
Anionic chains of Q2 terahedra terminated by Q1 - metaphosphate with a pyro end-- synthon?? perhaps??

Metaphosphate glasses
First Graham's salt, formed when NaH2PO4 is heated- consists of chains -commercially known as Calgon, an ion exchange water softener. The composition of metaphosphate glasses approximates to M2O.P2O5, (M is a group 1 alkali metal e.g. sodium) Consists almst entirely of Q2 units. Effect of replacing alkali metal oxide with alkline earth metal (eg. substitute MgO for Na2O ) is to increase the durability of the glass.

Ultraphosphate glasses
Sensitive to moisture - like P2O5 - some need to nbe kept in sealed ampoules.

Ternary phosphate glasses
Na2O/CaO/P2O5 glasses much studied as biologically compatible.

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=Ultraphosphates (Article)= Ultraphosphates are a group of phosphorus oxyanions that contain a higher proportion of phosphorus than metaphosphates. Lanthanide ultraphosphates, LnP5O14 have optical properties that may have industrial applications Ultraphosphates in common with other condensed phosphates (metaphosphates and polyphosphates) are made up of corner sharing {PO4} units. Where ultraphosphates differ is that they contain {PO4} units that share three corners. The number of shared corners in the {PO4} tetrahedra is sometimes designated as Qi where i is the number of corners shared. P2O5 contains only Q3 units in all of its forms (molecular P4O10 and polymeric). Ultraphosphate compounds contain both Q3 and Q2 units, whereas metaphosphate compounds contain only Q2.

Examples of ultraphosphates
Compounds are known that contain anions with the general formulae of (P2O5.(PO3)n)n– where n = 2 - 6, P4O112–, P5O143–, P6O174–, P7O205– and P8O236–. The stoichiometry of the anion is no guide to the structure. For example in the lanthaide ultraphosphates, LnP5O14, the phosphate anionic framework adopts a number of different forms. In Na3Fe8O23 the anion 8O236– has a cage "molecular" structure.

P5O143– P6O174- P7O205– P8O236-

In the classification of phosphates as salts of acids with the formula mH2O.nP2O5 ultraphosphates have m/n < 1, and richer in P2O5 than the metaphosphates ,which are salts of hypothetical acids formulated as H2O.P2O5 with anions PnO3nn–.

An ultraphosphate is a phosphate that is richer in phosphorus pentoxide than the metaphosphates. Examples of some known ultraphosphates showing the proportions of phosphorus pentoxide along with metaphosphate as a comaparison.

Some known ultraphosphates include

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