Page 89 - Materials Science and Engineering An Introduction

3.6 Polymorphism and Allotropy • 61

M A T E R I A L O F I M P O R T A N C E

Tin (Its Allotropic Transformation)

nother common metal that experiences an al- (or a) tin, which has a crystal structure similar to that
Alotropic change is tin. White (or b) tin, having of diamond (i.e., the diamond cubic crystal structure);
a body-centered tetragonal crystal structure at room this transformation is represented schematically as
temperature, transforms, at 13.2 C (55.8 F), to gray follows:

13.2°C
Cooling

White ( ) tin Gray ( ) tin

The rate at which this change takes place is extremely
slow; however, the lower the temperature (below
13.2 C) the faster the rate. Accompanying this white-
to-gray-tin transformation is an increase in volume
(27%), and, accordingly, a decrease in density (from
3
7.30 g/cm 3 to 5.77 g/cm ). Consequently, this volume
expansion results in the disintegration of the white tin
metal into a coarse powder of the gray allotrope. For
normal subambient temperatures, there is no need to
worry about this disintegration process for tin prod-
ucts because of the very slow rate at which the trans-
formation occurs.
This white-to-gray tin transition produced some
rather dramatic results in 1850 in Russia. The winter
that year was particularly cold, and record low tem-
peratures persisted for extended periods of time. The Specimen of white tin (left). Another specimen disinte-
grated upon transforming to gray tin (right) after it was
uniforms of some Russian soldiers had tin buttons, cooled to and held at a temperature below 13.2 C for
many of which crumbled because of these extreme an extended period of time.
cold conditions, as did also many of the tin church (Photograph courtesy of Professor Bill Plumbridge,
organ pipes. This problem came to be known as the Department of Materials Engineering, The Open University,
tin disease. Milton Keynes, England.)