Where does vacancy collide?

An empty site in a crystal where an atom is missing from the regular lattice, one of the basic point defects that let atoms move through a solid. Frenkel's 1926 Über die Wärmebewegung in festen und flüssigen Körpern proposed that thermal motion knocks atoms out of their normal positions and leaves empty sites behind, and Wagner and Schottky's 1930 Theorie der geordneten Mischphasen worked out how the number of such empty sites is fixed by temperature in a crystal at equilibrium. The referent is a hole in the atomic arrangement of a material, counted as a fraction of the lattice sites, and it has nothing in common with an open job in a firm or a run of moves through a set of positions.[1][2]

Unit

One missing atom at a single lattice site, given as a fraction of all sites

Recovery means

Vacancies migrate and are absorbed at surfaces, grain boundaries, or dislocations, which removes them from the lattice

Measured by

Estimated from the formation energy and temperature, and detected by positron annihilation or by changes in density and electrical resistivity

An unfilled job that an employer is actively trying to fill, treated as a quantity that rises and falls against the number of people looking for work. Beveridge's 1944 Full Employment in a Free Society set out the inverse relationship between unfilled vacancies and unemployment that later carried his name as the Beveridge curve, and Mortensen and Pissarides's 1994 Job Creation and Job Destruction in the Theory of Unemployment made the vacancy a firm's decision variable in a market where searching workers and searching firms are paired through a matching process. The referent is a posted but unfilled position in a labour market, counted as a rate or a running total, not an empty spot in a crystal or a link in a chain of moves.[3][4]

Unit

One posted job that no worker yet holds, counted as a total or as a vacancy rate

Recovery means

A vacancy ends when it is filled by a hire or is withdrawn by the employer

Measured by

Counted from job postings and employer surveys and reported as vacancy rates and job-openings series

An opening in a set of ranked positions that, once created, sets off a linked run of moves, because each person who takes the empty spot leaves another opening behind. Harrison White's 1970 Chains of Opportunity: System Models of Mobility in Organizations modelled how a single vacancy travels through an organisation like a token, passing from position to position until it reaches someone entering from outside or a slot that is abolished, and Chase's 1991 review Vacancy Chains carried the same idea into housing markets and even the shells that hermit crabs pass along. The referent is the empty position itself as it travels through a structure, traced as a sequence of moves, and it is distinct from a hole in a solid or a single unfilled job counted in a labour market.[5][6]

Unit

One empty position that passes from holder to holder, traced as a chain of moves

Recovery means

A chain ends when the vacancy reaches a new entrant or a position that is removed

Measured by

Reconstructed from personnel or housing records as the length and branching of the linked moves a vacancy generates